Parametric polymorphism is when a function's type signature allows various arguments to take on arbitrary types, but the types most be related to each other in some way.
For example, in Java one can write a function that accepts two arguments of any possible type. However, Haskell goes further by allowing a function to accept two arguments of any type so long as they are both the same type. For exampleAs a specific (and slightly more complicated) example, the well-known
map :: (a -> b) -> [a] -> [b]