After substantial weight loss, weight gain or simply from the effects of aging, skin on the upper arms can become inelastic, loose and flabby. An arm lift, also known as brachioplasty, removes that excess skin and fat and tightens and recontours the upper arm. If you have significant amounts of fat, you may want to consider combining the procedure with liposuction to reduce the thickness of the upper and outer arm also. For those with significant weight loss and excess skin on the chest or in the under arm area, tightening of these areas may also be indicated.
The procedure is done under modern light general anesthesia, usually as an outpatient, though, because both arms are involved, immediate post op hospital care at a nominal charge is available. Recovery is typically rapid, though you are somewhat restricted in arm use for a week or so. The scar is along the inside of the upper arm and hides completely with the arm at your side. Buried sutures that do not have to be removed also help minimize the scar.
The American Society of Plastic Surgeons provides more general information.