The gauges are in either pounds per square inch (psi) or Kilopascals (kPa). These measure the amount of air pressure in the primary and secondary air tanks.
The air pressure stored in the air tanks is the maximum available pressure for a service brake application. In other words, if there is 80psi in the air tanks, the driver can only make an 80psi brake application.
This fact is a common test questions on air brake knowledge tests.