There aren't traditional levels in the game. You receive skill points for completing objectives - such as exploring a large or dangerous location or collecting a certain amount of food. There are reoccurring goals, exploration goals, and challenge goals. There are also hidden goals which will award the player for making particularly difficult decisions. You can use skill points to advance skills. When a certain number of skill points are gained, the player will receive one stat point to use in boosting one of their four stats.
There are also hidden perks which can bestow a choice of skills onto the player when they reach certain milestones in the game.