Content Complete means that all the content the creators planned to put in the game is now officially in and can be accessed by the players. But there’s still one last thing to do, and that is fixing bugs and balancing things. The final build of the game, which will take it out of Early Access, will be released on February 2016.
The final act and ending to the story is now available, as well as the Elite and Ultimate difficulty settings, a second Roguelike dungeon (Bastion of Chaos) and a level cap increase to 85, among other things.
A forum post in the official community detailing all the changes is available here.