What is the function of the governor in an air brake system's air supply?

AAMVA CDL Manual, Section 5.1.2 (Air Compressor Governor), p. 5-1; 49 CFR 383.111(b)(1)

Knowing what the governor does helps a driver interpret the air pressure gauge during warm-up: pressure should climb, then hold steady once cut-out is reached, which is a normal sign the air system is functioning correctly before driving.