Despite its name, it has nothing to do with installing a cloth tube in your body.
Rather, it involves trimming off and removing a portion of the stomach. This leaves a smaller and more tubular stomach that is called a sleeve. The trimmed stomach can then become full from a smaller volume of food. Less food can mean less calories consumed and more weight lost.
The portion of stomach that was removed also takes with it hormone-releasing cells that might have a large effect on hunger. The end result can be substantial weight loss and improvement of serious weight-related medical conditions.