For someone skinny, you are likely to have a ectomorph body type. Your metabolism is very high, so you would have problem gaining weight if you consume "normal" diets.
Muscle building takes up alot of energy. You would need to consume extra calories - more calories than what your body needs a day - if you are looking to pack on significant muscles.
Mass gainers can help you attain those extra few hundred or thousand calories daily easily. A serving of mass gainer cab easily hit about 700 calories. However, not all mass gainers are created equal. Some are made up of poor quality ingredients which can result in you gaining much more fat than muscle because of the high simple carbs content. Moreover it is also not as healthy compared to eating more servings of solid food to get more calories.