I'm moving a light switch to a different wall and have found that the cable from the switch to the light has 3 wires but I don't know why. The 2 wire source feeds to an adjacent switch for a different light which then feeds this switch with 2 wires. The switch in question is a single pole. From it, there is 3 wire cable that leads to a light which then connects to other lights controlled by their own switches. There's no three way set up and this isn't a switch loop either. Plus, The two black wires are using the same terminal. What's going on here?
This circuit is being fed forward from this location.
The two blacks are constant hot, the red is switched hot.
If you follow the /3 when you get to the other end of it, the red is switched hot, the black is constant hot.
Edit to add:
If this switch is feeding a ceiling box, this is a common configuration so that the light switch (red wire) only controls the light portion of a ceiling fan, allowing the fan itself to be controlled separately by the pull chains. If there is currently just a light fixture, then they only used the Red switched hot wire, and capped the black. The circuit could also, optionally, continue forward from the ceiling box using the black-constant hot, and white-neutral.
My first idea is that you had in the past a double single pole light switch for independant switching of to light fixures.
I do not like the bare wire in the background. It should be properly connected with a connector of suitable format.