Experience is an in game mechanic that focuses on player progression. Right between the Mana points and Health and Stamina bars, there is a bar that is shorter than the Health and Stamina bars. This is the Experience bar, which increases as you kill monsters or animals. As you level up, the amount of exp you need increases. More powerful monsters and animals give you more exp, which makes it quicker to level up than killing the same beginning animals or monsters.
Each level the player gains requires more experience to level up.