A TPMS or Tire-pressure monitoring system monitors the air pressure inside all four tires and alerts the operator if any of the tires are not at the correct tire pressure. This is accomplished by utilizing a warning light, a gauge, a key chain or a display on the dash, so that the driver is made aware of any inflation problems before tire damage can occur. Direct TPMS utilizes sensors that are attached to the inside of the tire rim or sensors fitted at the back-end of the valve stems. These sensors relay inflation information for each tire via radio frequency.
For more info, see A Tire Pressure Sensor Buyer’s Guide