What do "extension" and "flexion" mean?

Extension:

Extension is the movement of body that increases the angle between two body parts. The elbow extension increases the angle between ulna and humerus. The knee extension increases the lower limb.

Flexion:

Flexion is the movement of body that decreases the angle between two body parts. The elbow flexion decreases the angle between humerus and ulna. The knee flexion causes the decrease an angle between tibia and femur.