An active layer means any pixel changes made (brush strokes, filters, erased pixels) will be applied to that layer. Visible means you can physically see the information on that layer. 

Layers can be active and invisible at the same time, and this is a common set up when doing some colour separations. For this reason, it is important to be mindful of what is visible and what is active in the Layers Palette at all times.

You can have images and layers active without being visible. In this case any changes you make will affect them, but you will not be able to see these changes until you make the layer visible. This practice is common when colour separating designs, but not common in the design process. 

A black dot inside the circle to the side of the layer indicates that it is visible. Individual images and layers can be made visible/active, you can view and activate multiple layers at the same time. It depends on what you need to do at any given time. 

Activating several layers in the Layers Palette

Click on the uppermost pencil and drag your cursor downwards, over all the layers you want to activate. The pencil icons will become dark to indicate that they are now active. 

Alternatively, click the pencil icon located above the layer tabs to activate or deactivate all the layers in the file. If you cannot see this icon you will need to edit your Layer Palette Settings . Open the Settings and click on the Show tab and ensure the Tools option is ticked.

If you need to activate layers which are not beside each other of the Layers Palette, you can hold down ⇧ (shift key) as you click on the individual pencil icons. 

Making more than one layer or image visible in the Layers Palette

Click on the uppermost circle next to the pencils and drag downwards. Alternatively, click the eye icon located above the layer tabs to make all layers visible or invisible. Black dots will appear to indicate that the layers are now visible and the information contained on those layers will become visible in the Design Window.

How to deactivate a layer in the Layers Palette

Activating a layer will automatically deactivate any other layers which were active. However, if you wish to have all layers in your file deactivated, hold down ⇧ (shift key) on your keyboard and click on the active pencil next to the relevant image or layer to deactivate it.

How to quickly view one layer on its own

Hold down ⌥ (option key) on your keyboard as you click on the visible indicator of a layer. All the other layers will be made invisible and only the chosen one will be displayed.