A piece of climbing
gear used to climb up a rope.
The standard ascender system is called the Frog. It consists of two clips designed to allow the rope to slide when they move up, but lock when you put weight on them. All of the lifting comes from your legs.
One clip is attached to your harness.
The other clip is held in one hand, and has a length of rope or webbing with a loop at the end for your foot.
To ascend, you rest your weight on the harness clip, and slide the other clip farther up the rope. Then you stand up in the foot loop, putting your weight on the hand clip and sliding the harness clip up the rope. Rest your weight back onto the harness clip and repeat.