The acromion is a piece of bone that's part of the shoulder blade. It curves above the shoulder joint and attaches to the collar bone. A tendon from the rotator cuff called the supraspinatus goes under the acromion and attaches to the shoulder joint. When the acromion pinches against the tendon, it causes a painful condition called shoulder impingement.

Acromioplasty is the removal of a piece of acromion process and may be done as part of a repair of the rotator cuff. It takes the pressure off the supraspinatus tendon and relieves the pain. You can see an easy to understand picture of this at: http://www.doereport.com/generateexhibit.php?ID=4190