2 wires going into one terminal on a single pole switch?
The 2 wires are normally your hot coming in and then going on to another device the single wire is usually the switched hot to the light. I do not like back stabs; they usually hold up on lighting only circuits, but when the other wire is to a chain of outlets, all the power is going through that stab, a very tiny contact area. The stabs get hot and quit working and even melt switches but more often receptacles. I like to use a short piece of wire with a wirenut connecting the hot coming in and out and the short wire, or pigtail, to the switch.  A lighting circuit for a room is usually fine if one must use backstabs, but I prefer the screw terminals even though they take longer to install myself and the electricians I work with use pigtails.

It could be a single hot going out to 2 different lights, also, but the first scenario is the most common.