Haskell elem function definition has t a can you explain that

Question

I was looking at the type annotation of function elem. It looked like this:

elem :: Eq a => a -> t a -> Bool 

I understand that Eq a => is a type constraint. So a must support ==.

But I don't understand t a, where does this t came from ? What does it do? Why is it necessary?

Link: https://hackage.haskell.org/package/base-4.16.0.0/docs/Prelude.html#v:elem