It's actually called War Emergency Power (WEP).
Without getting too technical it's akin to hitting a super charger or Nitrous. It gives the engine additional boost for a finite period of time. You can use it to give you a ilttle more speed or maybe go up just a bit higher, if required.
Some planes have it and some don't. It's an easy check to find out if the one you're in has it. Hit your P key to toggle it on and off. You can actually hear the engine power increase and note it on the gauges.
Experiment with it. Take any plane into the air then hit it and leave it on. See how long it lasts in that plane. See how long you have to wait until you can use it again.
Hope this helps.
