How do functions work?

[Gloo pot]

Im just learning how to use functions properly but i kinda got no were.
To me this script look fine but obviuosly not because it dont work.
Can any one give me some insight on why?

onClipEvent (enterFrame) {
	r = function () {
		if (this._rotation != 90) {
			this._rotation += 5;
		}
	};
}
on (release) {
	r;
}