I understand that Haskell's filter is a high order function (meaning a function that takes another function as a parameter) that goes through a list checking which element fulfills certain boolean condition.

I don't quite understand its definition:

filter:: (a->Bool)->[a]->[a]
filter p [] = []
filter p (x:y) | p x = x:filter p y | otherwise = 
 filter p y

I understand that if I pass an empty list to the function, it would just return an empty list, but how do I read the last two lines?