Memory is an Attribute in Divinity Original Sin 2.
Memory is a new stat and is necessary to memorize Skills and only a limited amount of skills can be memorized at any one time. More powerful skills also require a higher level of memory.
Memory Slots determine the number and quality of skills you can use at the same time. You can learn every skill in the game, but only a limited amount can be active at any given time. Outside of combat, characters can organize their active skills to prepare for their next encounter. The more powerful the skill is, the more Memory Slots required.
Characters gain an additional Memory Slot every 2 levels. (2,4,6,8,10,12 etc.)