In the book, The Hunger Games: Catching Fire by Suzan Collins, What is a "Quarter Quell"?

So in the second book of the trilogy The Hunger Game (Catching Fire), Katniss and i think Peeta get called back into the arena. but i REALLY don't get what a Quarter Quell is and why it makes them have to go back in the arena. and i also think that in the chapter i was reading they said "On the 75th anniversary as reminder to the rebels that even the strongest among them cannot over come the power off the capital. The male and female tributes wil be reaped from their existing pool of victors." whats that mean??? please answer those all those questions for me pleeease! and thanks :] THE HUNGER GAMES ROCK