I am a beginner in Photoshop and trying to emulate a template, but now I'm stuck. I don't know how to name the procedure marked by the arrow. So I have a picture which is masked by another layer. Can you tell me how to term it, in order to find a related tutorial or tell me how to do it?
The "square" layer is a Shape Layer. The "picture" layer is a Smart Object. I've created a similarly structured document, to demonstrate this.
I created a black rectangle as my Smart Object and placed a rounded rectangular Shape Layer below it.
Right-click (CTRL + Click on Mac) on the Smart Object (your "picture" layer) and click on Create Clipping Mask.
You will notice that the Smart Object I created (your "picture" layer and my "Layer 1") will now have the Shape Layer beneath it applied, as a Clipping Mask.
If you were to right-click on your "picture" layer and select Release Clipping Mask from the menu, you will see that the Smart Object will lose the applied Clipping Mask and be displayed at its normal aspect ratio.
The little bent arrow means it's attached to the layer below and using it as a clipping mask.
I'm simplifying here, but it has the effect of only showing the pixels in the upper layer that overlap the pixels of the lower layer.
Choose Layer-->Release Clipping Mask to make it a normal layer.