Output compare is the ability to trigger an output based on a timestamp in memory, without interrupting the execution of code by a processor or microcontroller. This is a functionality provided by many embedded systems. The corresponding ability to record a timestamp in memory when an input occurs is called input capture.
References Microchip Documentation on Output Compare: DS39706A-page 16-1 - Section 16. Output Compare http://ww1.microchip.com/downloads/en/DeviceDoc/39706a.pdf