A cel (from celluloid) is one image in a specific frame and layer, at a specific xy-coordinate in the canvas.
The difference between a frame and a cel, is that a frame is the set of cels for all layers in a specific time:
On RGB images each cel has its own opacity level. You can use the slider at the status bar to modify its opacity:
You can do the same right-clicking a cel and selecting Properties option (or Frame > Cel Properties menu):