How does your posture amount to proper posture?

Good posture is important to balance: by standing up straight, you center your weight over your feet. This also helps you maintain correct form while exercising, which results in fewer injuries and greater gains

some good posture:

chin parallel to the floor
shoulders even (roll your shoulders up, back, and down to help achieve this)
neutral spine (no flexing or arching to overemphasize the curve in your lower back)
arms at your sides with elbows straight and even
abdominal muscles braced
hips even
knees even and pointing straight ahead
body weight distributed evenly on both feet.