It's when the plane or the air going over and under the wing is not moving fast enough to provide lift and the plane just becomes a rock. Not a problem if you have enough altitude to get some speed back up, but at landing it would be imminent death. Tip stall from what I can tell is one side of the plane loosing lift and rolling off to that side.

Let me know If I'm right guys, I have been around planes and done about everything you can do except fly a real one myself.