Provides contant 12v power to your trailer.  2019 and newer Chevy/GMC and 2021 and nwer Ford and Dodge trucks may require this adaptor to run any camera if the have pulse width modulation.
These are as simple as plugging in between the trailer and your truck.  This also allows you to turn on your marker lights and power your cameras even when your RV is disconnected from your vehilce.