To add, t is typically some kind of container although it doesn’t have to be. For instance, “t a” could be a “list (of) cat(s)”, except it doesn’t have to be a list (can be anything that’s an instance of Foldable), and it doesn’t have to be cats (could be a giraffe or a piano or any other type).
In “list (of) cat(s)”, list can be thought of as a type-level function that takes a type (cat) to construct a concrete type (list cat), hence the notation.
This shows you commonly defined instances of Foldable (notice that the very first one is a list, denoted by [ ]).