A piece of
rappelling equipment used to control your slide down the rope.
There are a number of different types, but they generally attach to your harness and provide friction against the rope, which you can control by manipulating either the rope or the descender. Letting up on the friction allows you to slide down the rope at a controlled speed.
The simplest descender is a figure 8 loop. You run the rope through one O, around the middle, and back through the O, with the other O attached to your harness. If you pull the rope at all, that is enough friction to hold you in midair. You descend by feeding a bit of rope through the loop with one hand.