A protein is like a pearl necklace. Each pearl represents an amino acid. Some amino acids can be synthesized by the body but 9 of them can not and therefore must be brought by food, they are called « essential amino acid ». These are: leucine, isoleucine, methionine, threonine, phenylalanine, tryptophan, valine, lysine.
In order for the protein synthesis to be optimal, they must contain simultaneously (at the same meal) the 9 essential amino acids. If one of these elements is missing, protein synthesis is limited.
Animal proteins (fish meat, seafood, egg and dairies) provide all the essential amino acids; they are called "High quality proteins". Plant proteins (cereals, legumes, vegetables, nuts) have for the majority at least one limiting factor. As they are deficient in one or more essential amino acids, they must be completed in order to optimize their absorption. For example, wheat is limited in lysine, while legumes are limited in methionine. Thus, it would be appropriate to combine them together during the same meal to have all the essential amino acids.