Nothing underneath it all
An odd thing: even foundations, literal foundations, aren't foundational in the sense that foundationalists want foundations to be. Philosophers often tend to forget that metaphors are metaphors, analogies, analogies, but in this case it's particularly odd because the metaphor itself is just wrong.
The idea is that ideas, understandings of the world, knowledge, etc. must be based on something, it must have a foundation, there be something solid underneath. Pretty much everything, though, starts from an assumption, or tautological definition, a founding axiom that, itself, doesn't have a foundation. An example is the idea that everything, in principle, can be proven to be true or false. But how, if you accept that, would you prove that idea to be true or false?
It only works as an assumption, or a trivial, definitional claim.
The foundationalists' question, though, kind of gives itself over to infinite regress, even though, ostensibly, that's what they're trying to avoid. The stock story is the one (Hindu?) of the question of "what holds up the world?" And the answer is "elephants." "But what holds up the elephants?" And the answer is "turtles." "And what holds up the turtles?" "More turtles." That this is nonsense is the nightmare of foundationalists, but what answer could they get? What answer did they did think they would get? What would be taken or could be taken as foundational? That is, what answer could possibly be given that wouldn't elicit the question again?
And think about actual foundations. What holds up a house? A foundation. And what holds up a foundation? Kind of nothing. There's dirt, but it isn't solid, which is why you need a foundation, which is big enough and solid enough to do the job, but the job is just supporting an edifice. It's not absolute or anything, and there's no foundation for the foundation, because that's not needed.