I believe it is mentioned in passing in several of the books, but if I recall correctly the Hydrogen Sonata and Excession went into more detail. He does discuss it in his notes on the culture[0]:

> We accept that the three dimensions of space we live in are curved, that space-time describes a hypersphere, just as the two dimensions of length and width on the surface of a totally smooth planet curve in a third dimension to produce a three-dimensional sphere. In the Culture stories, the idea is that - when you imagine the hypersphere which is our expanding universe - rather than thinking of a growing hollow sphere (like a inflating beach-ball, for example), think of an onion.

> An expanding onion, certainly, but an onion, nevertheless. Within our universe, our hypersphere, there are whole layers of younger, smaller hyperspheres. And we are not the very outer-most skin of that expanding onion, either; there are older, larger universes beyond ours, too. Between each universe there is something called the Energy Grid (I said this was all fake); I have no idea what this is, but it's what the Culture starships run on. And of course, if you could get through the Energy Grid, to a younger universe, and then repeat the process... now we really are talking about immortality. (This is why there are two types of hyperspace mentioned in the stories; infraspace within our hypersphere, and ultraspace without.)

> Now comes the difficult bit; switch to seven dimensions and even our four dimensional universe can be described as a circle. So forget about the onion; think of a doughnut. A doughnut with only a very tiny hole in the middle. That hole is the Cosmic Centre, the singularity, the great initiating fireball, the place the universes come from; and it didn't exist just in the instant our universe came into being; it exists all the time, and it's exploding all the time, like some Cosmic car engine, producing universes like exhaust smoke.

> As each universe comes into being, detonating and spreading and expanding, it - or rather the single circle we are using to describe it - goes gradually up the inner slope of our doughnut, like a widening ripple from a stone flung in a pond. It goes over the top of the doughnut, reaches its furthest extent on the outside edge of the doughnut, and then starts the long, contracting, collapsing journey back in towards the Cosmic Centre again, to be reborn...

> Or at least it does if it's on that doughnut; the doughnut is itself hollow, filled with smaller ones where the universes don't live so long. And there are larger ones outside it, where the universes live longer, and maybe there are universes that aren't on doughnuts at all, and never fall back in, and just dissipate out into... some form of meta-space? Where fragments of them are captured eventually by the attraction of another doughnut, and fall in towards its Cosmic Centre with the debris of lots of other dissipated universes, to be reborn as something quite different again? Who knows. (I know it's all nonsense, but you've got to admit it's impressive nonsense. And like I said at the start, none of it exists anyway, does it?)

[0]: http://www.vavatch.co.uk/books/banks/cultnote.htm