Compressor Repair checks for problems that could be preventing Qmaster from operating correctly.... Apple's Compressor relies heavily on a service called Qmaster in order to work. Qmaster allows you to split rendering tasks across multiple machines in a render farm but it is still required even if you are only rendering on a single computer.

The link between the two is so important that if the Qmaster service fails to launch, Compressor will be unable to submit batches. Should this occur, Compressor Repair checks for problems that could be preventing Qmaster from operating correctly. It checks for missing files, incorrect permissions, and attempts to manually start the qmasterd process.