Why Beans and tofu are considered good sources of protein but are incomplete protein sources?

Info source:

http://www.m.webmd.com/fitness-exercise/rm-quiz-protein-myths-facts?ecd=wnl_day_101512&ctr=wnl-day-101512_ld-stry


"Incomplete" protein sources are low in one or more of the essential amino acids. Beans and tofu are considered good sources of protein but are incomplete protein sources since they contain lower amounts of some essential amino acids.