Knowing yourself well means knowing your values in life, your beliefs, your personality, your priorities, your moods, your habits, your body, and your relationships.
It means knowing your strengths and weaknesses, your passions and fears. It means knowing your likes and dislikes, your tolerances and limits. Knowing yourself well means knowing your purpose in life.
Remember you are not born knowing yourself. You do not get to know yourself simply by growing up and growing old. Knowing yourself is a conscious effort, you do it with intention and purpose.