Release basically means to clear the cue list. It is a very common term on consoles that use more than one cue list or stack. Think of it as like when you release the button on a door bell. When you release it, the door bell turns off. So when you release a list, it turns off. It is also very important thing to keep in mind with tracking consoles and what lists are active and have priority (I am such a bad speller, I had to look at the word on the Pig to spell it...).
BO or Blackout means that the intensity channel on all fixtures that aren't parked or independent go to 0. The GM or Grand Master is kind of the same thing except it sets the highest intensity level of fixture and limits it base on the GM's value. For example, with the GM at 75%, instensity levels go up to 75% at max. Also note that it should base levels on what the fixture level is. So if I have a fixture at 50% and bring the GM to 80%, that fixure is now at 40%. Take it down to 50%, and the fixture drops to 25%.