It is a gametype created by AZN FTW and here is a breif guide to it right here.It is a really fun game and you must try it soon you will have a blast.Hope this helps dude.
Its basically territories that can be recaptured and captured until the round ends when they lock. kind of like tug of war but with halo.
Have you played Land Grab in Halo 2? It is pretty much the same concept. Basically, the territories are set up in a linear pattern; you have to capture the current battleground to advance further. Territories don't lock, and can be recaptured until the round or game ends.