Keyframe Layers allow you to blend between keyframes/ animation curves. This is very useful when you need to switch between animation states like running and walking. Keyframe Layers allow you to transition between these states without having to copy/paste blocks of animation.
Active - enable/disable the layer.
Strength - determine the strength of the layer.
Normal - keyframes on the layer are additive.
Overwrite - keyframes on the layer will overwrite other layers.