Face to path is the difference between the face angle and the club path.
For a right-handed golfer, a negative face to path would represent a face angle that is “closed” to the path (encouraging draw spin) and a positive face to path would represent a face angle that is “open” to the path (fade spin).
A zero face to path means the club face is pointing in exactly the same direction as the path ("square") promoting a straight, curveless flight.
Face to path is a key factor in determining the expected curve of a shot. Assuming centered contact, the ball should curve towards the face angle and away from the club path (if face to path is not equal to zero).
At any point in your journey of improvement, it adds necessary focus if you are clear on your intended ball flight (fade, draw, straight). You can then put a number on what good looks like, to inform your practice.
For a gentle 5 yard drawn 7 iron, the average golfer might look to a -2 deg face to path (+2 if a lefty). Matched with a +4 deg path, this would produce a lovely flight starting right of the flag and curving back to it.