That's a good question... you know when you are really exhausted and you just can't sit up straight any more.. your just too worn out, and you kinda slump over and want to take a nap at your desk?
It's kinda like that.... sort of.
Slumping glass can be explained this way. Visualize. a piece of glass, say 4" x 4". Now place the glass on the rim of a bowl (Kiln Form or mold). Now heat up the kiln slowly until it reaches temperatures of around 1200 - 1350 degrees. Hold the temp for 10 - 20 minutes. At this temperature the glass can't hold it's shape anymore and is going to start melting (so to speak), As it melts it will take on the form in the shape of the mold/form. Once you achieve the desired slump, the kiln and glass are cooled slowly.
That is what slumping is.