The word "dim" means "not bright". So the verb "dim ___" means to reduce the amount of light.
You can "dim the lights" in a room, which means to make the room less bright. Sometimes lights have a switch that allows you to dim them without turning them off. If the lights only have an "on/off" switch, then "dimming" the lights means tuning them off.
You can also dim things like a computer screen or TV.