http://halo4infinitychallenge.com/platform/web/registration/Welcome.html
So what is this? Well first go check that link as it should answer most if not all of your questions.
It’s a challenge and you need to climb the leaderboards! The two grand prizes are a Halo truck and a 2 minute appearance in a Halo game. The truck can be won by winning the War Games portion of the Infinity Challenge, while the 2 minute appearance in Spartan Ops. While I’d love to win the truck, I REALLLLLLLY want to win that 2 minute spot. To be immortalized in Halo like that? WOW!
I wish everyone good games for those who participate!
-Sal