What does a pair of vertical pipes mean within a function, as related to computational complexity?

I'm reading Some connections between nonuniform and uniform complexity classes and trying to understand what the vertical pipes mean within h(|x|).

Let S be a subset of {0,1}* . Let h : N -> {0,1}* where N is the set of natural numbers. Define S : h = { w x : x is an element of S and w = h (|x|) } . Next let S be any collection of subsets of {0,1}* and let F be any collection of functions from N to N.


Solution 1:

I think the correct way to interpret this is that h is a function that maps a boolean string x by its length to another boolean string w in {0,1}*.