Early release in a golf swing happens as you swing your golf club down to impact. Release occurs when the angle that you create at the top of your backswing between your left arm and the shaft of your golf club, releases from the angle into a straight line along your left arm and the golf club shaft.
More consistent players tend to create this straight line as they strike the ball or just after impact. So early release happens, when you release the angle created between your left arm and the club shaft, into a straight line, before you get to impact.
To prevent early release during your golf swing, work on the following drills.
Initially, place the head cover from your driver on to the floor to the right of the golf ball and on the target line. Place it so that it is on the target line just outside your right foot. Take up your set up position and hover the club head up off the floor. Swing back to the top of your backswing so that you miss the head cover and then slowly swing down towards the ball. Work on striking the ground to the left of the head cover where the ball would be, without hitting the head cover. If you strike the head cover, you have released the club early. Work on swinging slowly and strike the ground on the left of the head cover, rather than striking the head cover itself.
Once you are doing this more successfully, begin to increase the speed of your swing gradually, until you are making a successful movement at a full swing speed. You can then reintroduce the golf ball and actually hit balls and if you are striking the ball without hitting the head cover, then you are releasing the golf club correctly.
Another drill that you can work on to prevent you from releasing the golf club early is to place an alignment pole across the target line about two feet to the left of the golf ball. Place it through a range basket or balance it on top of something that will allow the pole to be about three inches above the ground. Work on swinging the golf club so that you strike the ground where the ball would be and then hit the pole. If you release the golf club early, you will not be able to strike the pole as the club head will be on the upswing and will swing above the pole.
Work on either or both of these drills and you will begin to release the golf club correctly during your downswing, allowing you to strike the ball more consistently and achieve much greater distance.