All functions in haXe are typed, so for example, when you write:

public function outputString(st : String) : Void;

we know that the outputString function takes one parameter of type String and returns Void (that actually means that it does not return any object). This is represented as the following type:

String->Void

Now, imagine the following function:

public function sumAndCeil(a : Float, b : Int) : Int

Its type is:

Float->Int->Int

Ok, this was the easy part.

As mentioned earlier, a function can be given as an argument to another function. That also means that a function can take a function as an argument. This is useful, for example, to apply filters on lists or arrays. There's such a case in the standard library: List.filter. I will simplify things...