Complete, or high-quality proteins, are packed with specific amino acids. Amino acids are the building blocks of proteins. There are about 20 different ones. Your body needs them all but can only make some. The rest have to come from your diet. Those are known as "essential" amino acids. Complete proteins, like eggs, cheese, meat, soy, and quinoa, have all nine essential amino acids.
"Incomplete" protein sources are low in one or more essential amino acids. But you can still get all the amino acids you need even by eating a variety of incomplete protein sources.