Most trailers operate on a 2-wire system where the brake lights and turn signals operate on the same wire. When the towing vehicle has separate wires for brake lights and turn signals or a 3-wire system, a tail light converter is required. A converter is needed to take the separate signals from the vehicle and combine them so that the trailer lights operating on a combined system will work properly.
The converter you choose depends on the type of trailer you'll be towing and the car or truck you'll be towing it with. Be sure to consult your owner's manual for their recommendations.