Activision's thinking that a two year development cycle for Call of Duty games isn't enough so they're adding another into the fray. They're still going to keep a yearly cycle of new Call of Duty games. but each development team will now have two years instead of three to create one.
With that, Activision has brought in Sledgehammer Games to go with Treyarch and Infinity Ward in making it a three team rotation for the series. Sledgehammer Games has helped on past Call of Duty games so this is a promotion of sorts for them to be the sole developer on a Call of Duty title.
Will this create higher quality Call of Duty games in the future? Only time will tell. I will say I've enjoyed the Treyarch titles over the Infinity Ward titles the past few times though. We'll see how Sledgehammer Games does when they release their Call of Duty game.