"A complete protein (or whole protein) is a source of protein that contains an adequate proportion of all nine of the essential amino acids necessary for the dietary needs of humans or other animals"

http://en.wikipedia.org/wiki/Complete_protein

Many plants have "complete" amino acid profiles but one or more amino acids are too low to be completely adequate for humans. In this case, another protein source with a supplemental amino acid profile is required in order to balance out the deficiency in the first.