Posture is the position in which you hold your body upright against gravity while standing, sitting or lying down. Good posture involves training the body to stand, walk, sit and lie in positions where the least strain is placed on supporting muscles and ligaments.
It is especially good to think of your posture when you walk. If you hold yourself correctly whilst walking you will exercise the muscles that maintain proper posture. You will then not need to think about your posture so much at other times.