While you can select multiple layers in the Animation Layer editor, you can only set keyframes on one layer at a time. The layer that receives keyframes is the current layer.
Select multiple layers when you want to see the keyframes set on them at the same time, or when you want to re-organize the layers in the Animation Layer editor.
To select one layer:
The selected layer becomes bright green because it is also the current layer.
To select multiple layers:
Do one of the following in the Animation Layer editor:
The selected layers that are not the current layer are a soft green in the Animation Layer editor.
To select the current layer:
Do one of the following:
The current layer is a bright green in the Animation Layer editor.