Why isn't this variable unknown in the type definition?

Question

import Data.Vect

data NamedVect : Type -> (names : Vect n String) -> Type where
  Nil : NamedVect a []
  (::) : a -> NamedVect a ss -> NamedVect a (foo :: ss)

aa : NamedVect Nat ["name", "age"]
aa = [1,3]

I'm interested in the foo variable. Where is it from? Any documentation is appreciated.