This is a measurement of the direction of the clubhead at impact relative to the target line (independent of where the clubface is pointing). Most golfers relate this number to hitting the ball “in-to-out” or “out-to-in”.
A positive value means the club is moving to the right of the target at impact (“in-to-out” for a right-handed golfer) and a negative value means it is moving to the left of the target (“out-to-in” for a right-handed golfer).
To hit a straight shot, the club path should be zero. The club path is part of what influences the curvature of the shot. It also is part of what determines the ball’s starting direction.
An “in-to-out” club path is necessary to hit a draw and an “out-to-in” club path is necessary to hit a fade. The optimal club path depends on the type of shot the golfer wants to play. A golfer may want to hit a 5 yard fade, straight shot, or 10 yard draw. Each of these shots has its own optimal club path.