Your body needs protein, and lots of it. It is the building block of muscle, which is why eating a high protein diet with high protein foods is EXTREMELY important for anyone looking to increase muscle, gain weight/get big, or anyone just looking to build muscle. Without enough protein in the body, muscle mass will not increase. During your weightlifting and strength training workout, your muscles are getting broken down. In order to rebuild and repair your muscles, your body needs protein. It is as simple as that.
So now your wondering how a high protein diet can be effective for someone trying to lose weight and lose fat, right? Well, it is just as simple. Most high protein foods are extremely low in carbs and extremely low in saturated fat (which is the fat that you DON'T want to eat). Therefore, by eating a high protein diet loaded with high protein foods, at the same time you'd end up eating low carb foods and low saturated fat foods. And, if you didn't already know, in order to lose weight and lose fat, eating low carbs and eating little or no saturated fat is a MUST.
Some high protein foods are:
chicken
turkey
lean meats and beef
fish (just about every fish is high in protein, anything from tuna fish, to salmon, to shrimp)
egg whites
There are many other high protein foods, but these are the basics.