Replacing a single pole switch that has an extra backstab cable, with a dimmer switch
The backstab wire is a continuation of the hot ,black, wire probably to or from an outlet or main panel, Just wire nut it to the black wire from the screw terminal right next to it and the black wire from your dimmer.
The exposed bare copper wire is the ground wire.
The white, neutral, wire normally isn't needed when wiring regular dimmers or regular switches. It goes straight from the switch box to the fixture without being switched. If you were wiring up a smart switch, you would more than likely need the neutral.
Turn off the power before doing any work.