Meaning of `specialize`

Stream: new members

[Will Bradley]

What does the specialize attribute do? I looked at #help attr specialize but there's not much explanation. I don't know anything about metaprogramming either so reading the source didn't help. I know types are erased in compiled code, but does it run an extra compiler optimization pass after the type arguments to the function are filled in? Does it unfold/look inside functions passed as parameters, specializing on those?