The trailer brake controller is used for the brakes on the trailer your towing. It sends, I believe, electrical signals to the trailer. By adjusting the +/- it send the amount of pressure to be applied to them. When you press the brake pedal in the truck, it send the signal to the trailer brakes. The harder you press the harder the trailer brakes are applied. The trailer brakes and the truck brakes should grab at the same time.
By squeezing the trailer brake controller, it applies the amount of pressure you sent to it. The higher the number the harder it stops. Different size trailers will have different weights.
Go about 15mph then squeeze the trailer brake controller. The brakes on the trailer should stop the truck without the truck brakes. Then your set. I assume that it would remember the trailer you’re towing buy setting the profile. I only have a 1 profile for a travel trailer.