When an image has more than one layer, you must select the layer you want to work on. Changes to the image affect only the selected layer (also called the active layer). You can select only one layer at a time.
To select a layer:
On the Layer palette, click the name of the layer to highlight the selected layer.
Deleting Layers or their Contents
Expanding and Contracting Layers and Layer Groups
Viewing the Contents of Layers