Summary
While performing a regular jump, if your character is against the back wall, hitting Forward Flip will allow the character to jump off the wall.
Off the Wall
All Wall Jumping really affords the character is a little extra boost in height and the ability to alter the direction of their regular Jump if they are jumping straight up or backwards near the wall. The good thing about Wall Jumps is that it is considered a completely separate action from Air Dashes and Double / Triple Jumps, so it does not eat into the allowed amount of those actions. So in the case of a character like Chun Li, she can actually Jump from the ground, Double Jump backwards, then Wall Jump, and then Air Dash. The Double Jump and Air Dash take from her pool of two extra movement actions in the air, but the Wall Jump does not.