A reverse pivot is a position at the top of your backswing where you have the majority of your weight incorrectly on your left side (for right handed golfers).
It sees your spine angled towards the target so that the top of your spine is closer to the target than the bottom of your spine and it causes a variety of issues for you getting the club head back down to strike the ball. From this position at the top of your backswing, you may make a chopping action with your arms and the golf club as your body is already positioned over your left foot and so is unable to move left. Your downswing, therefore, becomes very dominated by your arms and as a result of this, you will swing the club head down towards the top of the golf ball, producing a low shot without much distance.
The alternative to this movement is that your weight shifts back to your right side as you hit, which again is incorrect, and results in a loss of power as your body is moving in the opposite direction to the way the club is being swung and you will tend to hit the ground before you strike the golf ball as your centre of gravity is moving incorrectly to the right and therefore, the lowest point of your swing arc is too much to the right of the golf ball.
The correct position to be in at the top of your backswing is to have your weight even between your left and right side, or just slightly on your right. Your spine angle should be tilted away from the target, with the upper part of your spine further away from the target than the lower part of your spine.
In order to correct this reverse pivot swing position, work on the following drill. Imagine you have a friend stood in line with you but to your right. They are holding their left hand out to shake yours. From your set up position, shake your left hand with theirs. This will see you rotate your upper body to the right as you extend your left arm to the right. Your weight will shift slightly right as your centre of gravity moves to the right and your spine will tilt slightly over towards the right also.
Work on this shake hands movement and you will achieve a much improved position at the top of your backswing that will see you hitting longer and much more consistent golf shots.