You control one member of a submarine crew exploring the depths of Europa, a moon of Jupiter. You'll be mining resources, battling off hideous monsters and trying to repair your ship when it inevitably breaks. You can play it singleplayer with bots filling the other crew roles, but the meat of the game will be in multiplayer, where you'll work with friends or online strangers to survive.
Andy's preview from earlier this week is a good place to start if you want to know more: as he says, it could create some amazing stories if the devs get it right, and the creation tools—which let you create ships, missions and monsters—look robust.
The Early Access release date was accompanied by a new trailer, at the top of this pose, which runs through the game's features in detail. Some of the footage looks choppy, which is worrying, but hopefully the devs can sort out the frame rate before release.