You “know” – or understand – a concept when you can convincingly answer these questions:
Do I grasp its definition? Be able to identify what kind a thing (or class) it refers to? How it is similar to other members of that class? How it is different from other members of its class?
Do I grasp its intuitive content? Can I give concrete examples of it?
Do I grasp why this concept is needed? Can I give examples of some naming needs for this concept?
Do I grasp in what contexts this concept plays a role? Where do we use this concept?