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 can be used to quickly set the opacity of selected layers.
To change the opacity of a layer:
From the Layers panel, select one or more layers.
Use the Opacity control to set an opacity value.
To use opacity quick keys:
Select a single layer or multiple layers.
Press a numerical key, or two numerical keys in quick succession, to set the opacity. For example: