The faces of woods are not uniformly flat, but slightly curved. The curvature from heel to toe (horizontal) is called bulge, while top to bottom (vertical) curvature is called roll.
Bulge compensates for the golf ball spin imparted by off-center hits. For instance, a shot hit off the club’s toe will have left sidespin; bulge makes the ball start farther right than a shot hit dead-center. Conversely, a heel shot will spin right; bulge makes it start left.
Roll affects the loft of a shot depending on where the ball strikes the face. A ball hit high on the face will travel higher than a center strike, while a ball hit low on the face will fly lower.