[Haskell-beginners] definition of combinator

Although I can use libraries like Parsec, I don't really understand what a combinator is, theoretically. There is an article here http://en.wikipedia.org/wiki/Combinator with the statement "A combinator is a higher-order function that uses only function application and earlier defined combinators to define a result from its arguments." Okay, so I believe a "higher-order function" is a function that takes function(s) as its argument(s). I don't know what "uses only function application" means. Application of what functions to what? Can someone give a concrete example that's simple? Thanks, Mike