During a sleeve gastrectomy, most of one side of your stomach is removed so that you are left with a smaller 'sleeve' of stomach, which is essentially a narrow tube. This remains connected to your oesophagus above and the first part of your intestine below. So, again, the size of your stomach is made smaller, meaning that you can only eat a much smaller meal. You will also feel fuller for a longer period of time after you have eaten.
The video below will give you a detailed account of what Sleeve gastrectomy surgery entails.