This is something new for the Halo franchise: Episodic content which may fill the gap between Halo 4 and Halo 5.
Taking place 6 months after the events in Halo 4, this new content is based on the planet Requiem and will consist of 10 episodes, each with 5 missions, that you can play with up to 4 friends. The first season is free and they recommend that you finish the campaign before to avoid spoilers.
More info in this article.