The attack angle in golf, is the vertical angle at which the club head is moving at impact.
A positive angle would mean hitting up on the ball, while a negative attack angle would mean hitting down on the golf ball. Shots off the ground should have a negative attack angle in order to ensure that the golfer is optimizing trajectory.
In order to maximize distance with a driver golfers must have a positive attack angle to hit up on the ball. This will not guarantee distance, but instead pairing the positive attack angle with the driver's loft and club speed will result in further distance.
The specific angle will depend on the type of club and skill level of the individual golfer. Different clubs have different angles that will maximize trajectory for distance and players can take advantage of using these different angles. The average angle for men on the PGA tour when using a driver was -1.0 degrees, and -4.1 degrees for the 6 iron. For women on the LPGA tour the average attack angle used driving was +2.0 degrees, and -2.3 degrees when using a 6 iron.