The optional second parameter is Unity layer you want the asset to be associated with. So now Unity has a reference to this sprite sheet: Next we need to define an animation to play. We can do this ...

The PSD Importer has many features and options that prepare the actor for animation. For example, enable the Mosaic option to have Unity automatically generate a Sprite sheet from the imported layers; ...

Learn how to use sprite sheets, which are images that contain multiple sprites or frames of animation, to optimize your 2D graphics rendering performance and create smooth animations.