Animation Layer Weight

Layer weight defines how much (normalized) animation from the layer will evaluate in the final output.

For example:

If you have have 2 layers "layer_1" with weight=0.25 and "layer_2" with weight=0.75. The final frame animation evaluation, will take 0.25 effect from "layer_1" and 0.75 effect from "layer_2".

For more info: https://docs.unity3d.com/Manual/AnimationLayers.html