Supplies air to the trailer reservoirs, and by doing so controls the application and release of the trailer's spring brakes.
The valve is an octagon-shaped red button on the dashboard.
If you forget how the button works you can always read what's written on the button: "Pull to Apply; Push to Release." Although somewhat cynical, reading doesn't always work because the text wears off over time.
Another way to remember is:
The parking brake lever in a passenger car. You pull it up to apply the parking brake and push it down to release the parking brake - the truck parking brake works in a similar manner.
When the driver pushes the button in, it fills the trailer air tanks and than the spring brake chambers with air and thus cages the parking springs. Consequently the parking brakes are released. When the button is pulled out, the air in the spring brake chambers is released via the quick release valve and the spring expands and applies the parking brakes.