Question

Like the question says.

This was found in List[() => Unit] which was used to store callback functions.

I understand List[type] and that Unit is a return type of a function that doesnt return anything.

Answer 1

(T1,...,Tn) => T is the type of functions that take parameters of types T1 through Tn and return type T. So () => Unit is the type of functions that take no parameters and have return type Unit. Consequently List[() => Unit] is the type of lists containing such functions.