As the wing moves through the air there is a rotor effect off the tips of the wing. At cruising speed these rotors are relatively small and fall off and behind the wing harmlessly, well they reduce efficiency. As the plane slows this rotor gets a little bigger and starts hitting the wing, as it does this that portion of the wing stops creating lift starts at the tip and moves in . at some point the wing stops creating lift and one will drop off before the other which is a tip stall.
Some designs are more prone to this than others. a thick chord wing. short wing span and long front to back has more of a tendency to tip stall than a long wing with a short chord ie thermal gliders. Though there are many factors that influence it like wing thickness airfoil shape and overall weight of the model.

If you have a model that is prone to tip stall like a CAP 232 or a heavy warbird you avoid the tip stall buy keeping your speed up. Use as much runway as you can at least on the first couple flights on take off. Come in hot when you land until you are sure of the planes safe landing speed. You avoid it with experiance in knowing how to recongize what is happening and reacte correcty without over correcting. Usually what I see happen is someone does catch the tip stall but over corrects and snaps the model the other way.