Apperception is how your mind puts new information in context. You get a perception of a chair through your eyes, but apperception is how your mind relates it to chairs you've seen before.
We have many perceptions: information we take in through our senses, like "It's cloudy today." An apperception goes one step further by considering the perception in relation to things you’ve perceived in the past. "There's Julia" is a perception, but "Julia is my friend" is an apperception, because it's based on past experience. "My stomach hurts" is a perception, but "I might throw up" is an apperception. Apperception is a sophisticated mental process that keeps developing through our lives.