The Face Angle is the direction of the club face at the impact. The club face can aim to the right, left or straight at the target. This angle is primarily influenced by the wrists.
For right handed golfers (for left handed golfers is the opposite):
a) if the club face is pointing to the right of the target at the impact this is considerate a positive value;
b) if the club face is pointing to the left of the target at the impact this is considered a negative value.
The face angle is the most important factor if you want to determine the starting direction of the golf ball, hit the ball in the center of the club face ( sweet-spot or center of gravity) !
One does not have to achieve a perfectly square position at the impact, meaning the club face is aiming exactly at the target, but a player must to maintain a good relation between the club face and club path (composed by swing direction, attack angle and swing plane) in order to properly execute the 9 different ball flight trajectories in golf.
This is called D-Plane!
Which club in your golf bag influences the initial direction of the golf ball the most?
...the answer is your putter! 95% of initial direction comes from the face angle and not from the swing path..
Below you can find an example of launch direction. The club face is closed by 2 degrees and the club path is open 6 degrees to the target.
The ball starts straight at the target and then curves to the left. This is a hook shot.
In general the face angle has four times greater influence than the Club Path at impact in determining the ball's starting direction.