All layers, except Background layers, are transparent. Even if a layer is filled with pixels, such as an image, a fill color, or both, the layer itself retains transparency capabilities, allowing other layers below it to show through when pixels on the top layer are erased or masked.
You can give any layer a degree of transparency by adjusting its opacity. Whenever necessary, the Background layer can be converted from a non-transparent layer to a regular transparent layer.
Transparent layers are what provide the remarkable flexibility and creative potential of Photoshop—it’s important to understand how they’re working for you!
With the exception of “Background” layers (which are not transparent), ...