The missing word appears three times, so it must mean something. Which word completes the Dumbledore quote? "It is important to __ and __ again, and keep __, for only then can evil be kept at bay though never quite eradicated."
Dumbledore always tried to tell the kids that trying, even if you don't win all the time, is good enough. Keeping Voldemort away by never giving up and keeping the fight going just to keep things placated will suffice until the big fight.