Thursday means another Q&A!
A clipping mask is a layer that defines the visibility of other layers. you need at least two layers to create one. The bottom layer will dictate what is visible on the layer above. To create the clipping mask, Opt/Alt-click between the two layers. So, let’s say you have a circle shape on one layer and a texture spanning the whole layer right above it. If you create a clipping mask, the texture will only appear where the circle is. The layers remain independent, so you can use the Move tool to shift each one. You can have any number of layers clipped inside a single clipping mask. Just move another layer right above the topmost clipped layer, then Opt/Alt-click between those layers. Here we’ve used clipping masks to populate a set of flat shapes with photos and texture, adding depth and colour.