If you crossed the air lines, you could drive away but you wouldn't have trailer brakes. This would be very dangerous. Pull gently against them in a low gear to make sure the brakes work. Each trailer and converter dolly has one or more air tanks. They provide the air pressure used to operate trailer brakes.
Air pressure is sent from the air tanks to the brakes by relay valves.
The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. It is important that you don't let water and oil build up in the air tanks. If you do, the brakes may not work correctly. Each tank has a drain valve on it and you should drain each tank every day. If your tanks have automatic drains, they will keep most moisture out.