Keep Good Posture
This principle is perhaps more frequenlty stated as "keep weight underside" so if you've seen if phrased this way, don't be confused. The idea of both is about balance and stability. If you maintain good posture you will by default be keeping weight underside. Keeping good posture (and keeping weight underside) must be maintained not just when standing or sitting still but also while moving.
The Japanese word for posture is "shisei", but this word encompasses attitude as well as just posture. So posture refers to balanced and stable attitude in addition to body mechanics.
Keeping good posture is ultimately about using energy effectively. By keeping good posture we can move naturally and comfortably without wasting energy. Once again another seemingly simple concept that is easier discuss than to do.
To your practice!