This adapter it used to convert a truck that has a 4 flat connection to a trailer that uses a 6 round.  The 6 round has 6 pins.  The 6 round is a somewhat older style connector, but is still used by many trailers.  The 6 round connector is used because the trailer has electric brakes.  The truck being used only has a 4 flat because it is designed to only run the lights on the trailer and not the brakes.  Using this adapter will allow all of the lights on the trailer to work properly, but NOT the brakes.  There are extra wires that are on the adapter that can use to wire up the truck with an electric brake controller so that you can control brakes on a trailer.  Without using those wires ONLY the lights will function on the trailer.