Well, this isn't quite the glorious return of Master Chief that some have been hoping for. Halo Online is a new, PC-exclusive, free-to-play online shooter that will launch this spring in Russia. According to Halo Waypoint it's multiplayer-only, and uses a heavily modified version of the Halo 3 engine that's specifically optimized to run smoothly on lower-end PCs.
Halo Online takes place on a secret UNSC space station called Anvil, where Spartan IV soldiers train and test experimental technology. It's being developed by Saber Interactive, a Russian studio that previously contributed to Halo: Combat Evolved Anniversary and Halo: The Master Chief Collection, and it will only be available in Russia: The FAQ says it's "tailored for Russian gamers," whatever that means, and while the possibility of a wider release isn't dismissed outright, "any expansion outside of Russia would have to go through region-specific changes to address player expectations."
I would guess that those "expectations" relate first and foremost to the use of the aged Halo 3 engine. It might be a smart way to attract new fans who aren't necessarily dedicated to being on the cutting edge of PC hardware, but it may not fly as well in markets where sub-par visuals can be the kiss of death. The FAQ also states that Halo Online was "built from the ground up specifically for the PC," and that there are no plans to bring it to the Xbox One. The closed beta is scheduled to take place this spring, but a launch date has yet to be announced.