Pegs are a special type of layer that do not contain any drawing. They are used strictly to offset and transform drawings that are under their hierarchy, without transforming the drawings directly.
When rigging or setting up a scene, it is recommended to add parent pegs for each of your drawing layers. This allows you to keep animation keyframes and drawings on separate layers, making it easier to work on the position and exposure of your drawing layers independently in the Timeline view. It also makes it easier to create a hierarchy of which body parts can be animated together and independently.
If you want to animate only on pegs, you can activate the Peg selection mode of the Transform tool in the Tool Properties view. You can also disable animating drawing layers, so that only pegs can be animated—see Disabling the Animation on Drawing Layers.
NOTE It is also possible to make a drawing layer the parent of another drawing layer. Just like the way animating a peg animates its children layers, animating a drawing layer with children will also animate its children layers. Both layers will still appear in the animation.