Depends on the amp but in most cases, its a remote gain/level control.
Lets say that your HU dose not have a sub level control. You set you amp's main gain as usual, connect the remote gain control and with that,
you can turn your sub level up a tad or down depending on what you want from the driver's seat.
DO NOT crank the main gain all of the way up and then use the rmt control. Set the main gain properly firs and then use the remote for
convenience.