It might be a sophisticated medical device, but the basic concept is easy to understand.
Our stomachs are large pouches that can normally hold a large volume of food. The gastric band can be placed around the stomach to form it into a gentle hourglass shape. As food is swallowed, is has to pass through the narrowed portion of the stomach. Patients often feel full much sooner during meals. Less food can mean less calories consumed and more weight lost.
Modern bands are also adjustable, which means that bands can be tightened or loosened in the office if needed. The end result can be substantial weight loss and improvement of serious weight-related medical conditions.