A brake controller is usually an oem or aftermarket installed device or module. It is mounted to the tow vehicle's driver's-side dashboard area, and engages a trailer’s braking system either time delayed, or in proportion to the tow vehicle's brake engagement when slowing down or coming to a halt. A brake controller is not needed with a trailer surge braking system unless using modern electric over hydraulic devices.
Most basic brake controllers will generally have a +/- gain adjustment. The tow vehicle operator would set the gain as high as possible but without the trailer brakes locking-up after making a few test stops. The heavier the trailer, the higher the gain adjustment would be set and therefore the less chances of wheel lock-up.
A wide range of trailers will contain trailer brakes (for example; larger boat trailers, horse trailers, covered utility trailers, enclosed trailers, travel trailers including small 10-foot and longer tent trailers and car carriers). Smaller trailers may not contain trailer brakes (for example; basic 4'x8' utility trailers). It is highly recommended that, if the total trailer weight is over a couple thousand pounds, the trailer have some sort of braking system, and the tow vehicle be equipped with a brake controller.