A layer's opacity determines how see-through a layer is and how much of the layer beneath is obscured or revealed. At 1% opacity, the layer is almost transparent, whereas at 100% the layer is completely opaque.
Numerical keys on a hardware keyboard can be used to quickly set the opacity of selected layers.
To change the opacity of a layer:
On the Layers panel, select one or more layers.
Do one of the following:
Tap the Opacity field and set a value via the flyout.
Tap Layer Options and use the Opacity control to set a value.
To change layer opacity using a hardware keyboard:
On the Layers panel, select one or more layers.
Press a numerical key, or two numerical keys in quick succession, to set the selection's opacity. For example: