"snap" generally refers to snap-rolls. It is a violent manuever where you induce a stall on on side of the wing. The resulting roll is fast and it's usually easier to do 2-3 than it is to do one. Normally, you can make a plane snap by simultaneously appling full up elevator and full rudder and maybe some aileron. It's a high stress manuever so don't do it unless your planes built for it.

The snap you're being warned about is a little different because it's unintentional. It usually occurs at slow speed during landing and again is caused by one side of the wing stalling before the other. The resulting roll is abrupt and often leads to a crash. Aerobatic models will often have the tendency to do this if you don't keep the speed up. The problem is that in order to have an aircraft capable of aerobatics some undesireable tendencies result including snaps.

I have a Super Decathalon that does this. On the third flight I learned a nasty lesson about airspeed and snaps that resulted in me rekitting the model. I rebuilt it and an cautious about keeping some speed on now during the landing approach. It's a great flying model. You just need to be cautious about an aircrafts shortfalls. Particularly when your close to the ground!