The building blocks of protein (of which our muscles, bones, hair, and fingernails are made) are called amino acids. Most of the 20 amino acids humans need are produced by our bodies naturally. But we must consume nine essential amino acids in the form of food.
All nine of these amino acids exist in meat, so non-vegetarians simply eat meat to fulfill their bodies' need for these vital building blocks. The good news for vegetarians is that the essential amino acids also exist in abundance in the plant kingdom. But vegetarians must draw on several different foods to build the same protein a non-vegetarian gets from meat. Since some but not all of the essential amino acids exist in certain foods, those foods are said to contain “incomplete proteins.”
Whole grains such as rice, for example, contain incomplete proteins. Legumes contain other incomplete proteins. Between the two of them, you'll find all the amino acids necessary for the human diet; so a plate of rice and beans meets the same protein-building needs in your body as meat would. Another such combination is wheat and dairy. Macaroni and cheese is a complete protein.
Vegetarians must remain vigilant about consuming a diverse diet, containing the building blocks for protein. Without them, muscle loss and weakness will slow down their bodies, and make them susceptible to all kinds of illnesses. Until recently, it was believed that complementary foods had to be consumed at the same time in order to build complete proteins. That myth has been debunked. It's now known that the body stores incomplete proteins (amino acids), and combines them for complete proteins when complementary ones arrive in the system, even if it's hours or even days later.
A secret weapon vegetarians have on their side is a superfood that contains all of the essential amino acids in a vegetarian source: soy. Soy foods such as tofu, soy flour, whole soybeans, and tempeh (a textured soy protein) are the virtual protein equivalent of meat. That's why soy products are such an important part of the vegetarian diet.