Mass gainer supplements work by providing a lot of calories in a shake, with the right proportion of macronutrients. If you are very skinny and have a very hard time gaining weight despite already eating a lot, go for the hi-calorie gainers. One serving of such gainers normally exceeds 1,000 calories. It is however recommended that you only consume up to 500 - 600 calories per dose. In other words, don't drink one full serving at one go. Split it over two periods in a day.