Protein in the context of nutrition is distinct from protein in the context of biochemistry. The former is really eight essential amino acids which are used by the body to manufacture the latter as needed.

All eight essential amino acids must be part of one diet in order to survive and are needed in a fixed ratio. A shortage on any one of these amino acids will constrain the body's ability to make the proteins it needs to function.

Different foods contain different ratios of the essential amino acids. By mixing foods that are rich in some amino acids with foods that are rich in others, one can acquire all the needed amino acids in sufficient quantities. Omnivores typically eat a sufficient variety of foods that this is not an issue, however, vegetarians and especially vegans should be careful to eat appropriate combinations of foods (e.g. nuts and green vegetables) so as to get all the essential amino acids in sufficient quantities that the body may produce all the proteins that it needs.