Like many RPG games The Witcher 3: Wild Hunt has extensive character development and levels. Unlike many games, however, gaining levels doesn't give very many character stat benefits by itself. Witcher 3's character development is almost entirely based on an unlock-by-level system; the higher your level, the higher the level (and therefore stats) of the equipment that you can wear and the more skill slots you unlock, on top of the additional skillpoint per level. So to understand this you also need to understand the Equipment, Abilities and Mutagens that fuel it.