Weight gainers are designed to add extra calories to your diet in order to help put on weight / lean muscle mass. It is critically important to take your weight gainer ON TOP of whatever you're already eating throughout the day. Eg do not replace your breakfast meal with a weight gainer shake. Have your breakfast meal as per usual, then add your weight gainer in mid-morning.
Please note, each gainer does contain different amounts of calories so please make sure you know how much is in yours before taking it!