Swinging over the top means that a player moves his/her back shoulder outward as the downswing starts and then swings from out-to-in through impact.  The result is usually a badly pulled shot or one that starts left and slices back to the right.  In practice, over-exaggerate the opposite move by keeping your back shoulder under as you swing into the ball.  Feel as if you are lowering your shoulder under your chin and hitting the ball with an uppercut punch.  Head to the driving range and practice some shots with this method until it feels comfortable and then return to your normal swing.