When banks can't borrow, they can't lend.
A general problem of that sort is a credit crunch, in which businesses and households can't obtain the credit they need.
That, in turn, leads to an economic slowdown or even a recession.
And in the worst case, banks that can't borrow go bust because they're unable to repay their own debts as they fall due.
That's why today's offer of three-year loans to the eurozone's banks by the European Central Bank matters, because it reduces some of the risk that there will be an unfortunate accident for a big eurozone bank in the coming 12 months, as eurozone banks have to repay some 700bn euros ($920bn; £580bn) of maturing debt over that period.
The background to all this, as you know, is that eurozone banks in general have been finding it harder and more expensive to borrow euros in recent weeks and almost impossible to borrow dollars, because US money market managers, who control the world's biggest pools of dollars, have decided there are safer places to keep their cash.
The consequence has been that the European Central Bank and central banks in eurozone countries have played a much more important role in lending to commercial banks.
As I've mentioned here before, the dependence of some eurozone banks on loans from these central banks can be seen as evidence that these banks have already failed, in that they would be bust if it weren't for massive loans from these taxpayer-backed institutions.
That's true in Greece, Cyprus, Portugal and Ireland, and to a growing extent in Spain and Italy too.
But the eurozone's authorities and investors are indulging in what may be a conceit, to the effect that this kind of exceptional semi-nationalisation or socialisation of banking is a temporary phenomenon, and therefore not as serious as it seems.
It's certainly an important phenomenon. Default by any of these big banks would have increased the gravity of the eurozone's debt crisis in a very considerable way.
In recognition of that danger, the European Central Bank 10 days ago not only said it would provide unlimited quantities of three-year loans to banks in two separate auctions. It also said that it and the national central banks would become less picky about the assets or collateral that commercial banks have to pledge to them in return for loans.
It is this ambitious provision of central banking credit to commercial banks, much more than anything agreed by the eurozone's political leaders at the last European Union council, which has brought a bit of calm to markets in the past few days.
The resonant point is that banks that borrow from the European Central Bank are encouraged by their respective national governments to use some of the proceeds to buy the debt of those governments; to, in effect, lend to those governments.
So, more than anything else, it is the lessening of funding constraints on Spanish banks that made Tuesday's auction by the Spanish government of its debt far more successful than it would have been just a few days earlier.
This is where we get to both the good and the bad of this very substantial programme of help for commercial banks by the ECB and the eurozone's national central banks.
The good is that the initiative undoubtedly buys some time for the eurozone's leaders to get to grips with the eurozone's fundamental problem, which, as the governor of the Bank of England points out, is a solvency problem relating to the capacity of some eurozone governments to repay their substantial debts.
The bad, as you know, is that there are only modest signs of the eurozone's government heads making progress in that respect.
And then there are some technical issues, which I find gripping (but I apologise in advance if you find what follows less racy than drying paint).
First is a sort of theological question: why does the European Central Bank refuse to lend directly to eurozone governments in any size, for fear that's the road to monetary and fiscal ruin, but says it's OK for it to lend to eurozone banks, which in turn lend to eurozone governments, and then use the eurozone government debt they buy as collateral for yet more loans from the European Central Bank?
Or, to put it another way, why is it OK for the European Central Bank to lend to Italy and Spain indirectly via the banks, when it's heresy to make direct loans?
Frankly, it can't seriously be argued that in behaving in this way the risks for the European Central Bank, and for the taxpayers that stand behind it, are lessened.
Because the whole reason that eurozone banks are perceived to be weak at the moment, and why commercial lenders are shunning them, is that these banks are seen to have excessive exposure to sovereign borrowers who may not be able to repay all they owe.
Or to put it another way, the ECB is taking a double solvency risk: on the solvency of the bank to which it lends and on the interconnected solvency of the sovereign standing behind said bank, and to which said commercial bank has lent.
I put this to a senior European Central Bank official a few months ago. The eloquent reply? A shrug.
The second big technical issue is that the European Central Bank is in some ways encouraging the demolition of the unsecured lending market for banks, which, for banks at least, is the nub of the difficulties they face in borrowing.
Now, the European Central Bank won't lend to banks without taking security from the banks, which is the normal central banking way.
But the ECB is only having to do this on such a large scale because eurozone banks can't borrow from lenders on an unsecured basis.
Here's the thing. As more and more of a bank's assets are pledged to the European Central Bank and national central banks as collateral, that bank has fewer and fewer unencumbered assets. This means that any residual unsecured lender to that bank would perceive that there are fewer free assets at that bank that - in a crisis - would be available to repay unsecured loans.
Or to put it another way, the more that banks become dependent on secured borrowing, whether from central banks or other lenders, the more reluctant any investor will be to provide unsecured loans to those banks.
All this has a disturbing implication, which is that there will be an almost inescapable credit crunch. Some banks will face collapse at the moment that banks have pledged all their unencumbered assets to central banks and other lenders, because those banks will at that point have run out of sources of credit.
Or to put it another way, the European Central Bank has provided temporary sticking plaster not a eurozone cure. You've heard that before, haven't you? But at least we may be able to have a Christmas unspoiled by the threat of banking meltdown.
Update 10:30: Eurozone banks have borrowed a huge 489bn euros for three years from the European Central Bank, which was at the top end of expectations.
There are two ways of seeing this result: either as confirmation of just how difficult it is for banks to borrow from conventional sources, and therefore that the sunny uplands of normal financial conditions are a long way off; or a reassuring realism on the part of banks that living hand to mouth on overnight loans from money markets is too dangerous.
A senior eurozone banker told me: "this shows you the money was really needed". Quite.