A polymorphic function is a function that works for many different types. For instance,

length :: [a] -> Int
-- here is a more explicit way to indicate that a can be any type
length :: forall a. [a] -> Int

You should think of the former signature as an abbreviation for the later one with the forall keyword.

This notion, that something is applicable to every type or holds for everything, is called universal quantification.

Higher rank types

With explicit forall, it now becomes possible to write functions that expect polymorphic arguments, like for instance

foo :: (forall a. a -> a) -> (Char,Bool)
foo f = (f 'c', f True)