Slumping is a technique in which items are made in a kiln by means of shaping glass over molds at high temperatures.
The glass is cut to the shape of the mold (but slightly larger to allow for shrinkage) and placed on top of it, before the kiln is heated.
The stages of the firing can be varied but typically start to climb at quite a rapid rate until the heat places the glass in an "orange state" i.e., flexible.
At that point, gravity will allow the glass to slump into the mold and the temperature is held at a constant for a period that is known as the "soak".
[1] An additional technique, used in the production of Roman pillar-moulded bowls, utilised a slotted tool to impress ribs on the glass sheet prior to slumping.