For each new limb, Limber generates three layers, which we call a set of layers: The limb layer itself, and two controllers that correspond to the position of the start (shoulder or hip) of the limb, and the end (wrist or ankle). The controllers are guide layers so that they don't appear in your final render. For simple IK animation, you’d parent the start controller to one of your character's body layers, and keyframe the end controller's Position property to animate that limb.
Controllers are shaped like a teardrop 💧 so that they are easy to grab in After Effects' composition panel, whilst displaying direction. The pointed end of the controller aims towards where it is rotated.
There is an effect called Limber on the end controller that gives you properties to determine the appearance and behavior of that limb. Some of those properties control aspects of the underlying, dynamic skeleton. Some of them control elements of the style of the limb layer.
You can use buttons in the Limber panel to add two other types of controller to a limb set - FK Controllers and Joint Controllers. Neither of these types of controller should be parented to other layers or have their Position animated by the user. Instead, they provide a layer in the timeline to which you can parent other layers, such as hands or feet, or… kneepads.