eggs themselves are a complete protein, so that's great. But for instance, beans have protein but you won't get the complete protein from them unless you eat corn with them. Eating nuts and/or seeds with beans also completes the protein. And so on.
Many of the ancient peoples knew this without having scientific labs to show it as we do. If you examine the every day diets of these folks you will often find they always ate one thing with another, thereby complimenting the proteins. (the ancient custom of eating legumes with corn tortillas, for instance)