More specifically, protein is made up of amino acids which are the building blocks for muscle. Muscle growth cannot be achieved if you don't supply your body with enough protein. Basically, if your car doesn't have gas, it won't go. Likewise, if your body doesn't have protein, it won't build muscle. Actually, if you deprive yourself of protein, your body will consume some of your lean muscle mass for the protein it requires.
Many foods have protein, but some certainly have more than others.
Foods like beef, fish, chicken, milk, eggs and cheese are rich in protein. However, it can be hard to get the amount of protein you need from whole foods without consuming too much fat. Variety in your protein sources is essential. One way to avoid fat and add variety to your protein sources is to use a whey or soy protein powder . If you workout, you need more than the average Joe.