You may have searched for:
background, bottom-most, collage, composite, foreground, level, mask, opacity, overlap, stack, transparency, transparent background
Layers are like stacked, transparent sheets of glass on which you can create images. You can see through the transparent areas of a layer to the layers below. You can work on each layer independently.

The image on the left has three layers. The photo is on the background layer, a semi-transparent rectangle is on the layer above the background layer, and the topmost layer contains the type.
Adobe Systems Incorporated