When you tow a trailer heavy enough to have its own brakes, a device in the tow vehicle coordinates them with your own braking. That device is the trailer brake controller. Here is what it does.
What it does
A trailer brake controller is a small unit (often mounted under the dash) that sends a signal to the trailer's electric brakes when you press the tow vehicle's brake pedal. It makes the trailer brake in proportion to the truck, so the whole rig slows together instead of the trailer shoving the truck.
Why it matters
A loaded trailer adds a lot of weight, and asking the tow vehicle's brakes to stop all of it causes overheating and fade - see why brakes overheat towing and brake fade. The controller lets the trailer's own brakes do their share, keeping stops safe and short.
Proportional vs time-based
Proportional (inertia-based) controllers sense how hard the truck is decelerating and match the trailer to it, which is smooth and responsive. Simpler time-based controllers ramp up trailer braking on a fixed curve. Proportional is generally preferred for heavy or frequent towing.
Part of a towing setup
The controller works with the trailer's electric trailer brakes and a good tow-vehicle brake setup; see towing brake upgrade guide. Keep the tow vehicle's brakes strong with quality pads and hardware.