What is aircraft c check?

The FAA in the USA requires a certain type and frequency of maintenance for all aircraft on an almost continuous basis.

Check A is done every month or 500 flight hours, typically overnight whilst parked.

Check B is done every 3 months, also overnight while parked.

Check C is done every 12-18 months or a specific number of flight hours determined by the manufacturer. This check requires the aircraft to come off the line and be inspected in a hangar environment.