The Weight of each animation layer determines how much of its animation plays in the result animation in your scene.
Keyframing the Weight value of animation layers lets you blend the effect of the layer in and out, animating how much the layer influences the result animation in your scene at any given time.
You can set the weight of animation layers using the weight keying buttons ( and ) or the Weight controls in the Animation Layer Editor, or by selecting the Layer node and editing the value in the Channel Box.
To keyframe the weight of animation layers
You can use the Time Slider, Graph Editor or Dope Sheet to view and edit the keyframes you set on the Weight value of animation layers. See View layer weight animation.