A legume is a plant in the family Fabaceae, or the fruit or seed of such a plant. Legumes are grown agriculturally, primarily for their food grain seed, for livestock forage and silage, and as soil-enhancing green manure. Wikipedia
Many legumes (clover, peas, beans, lentils, soybeans, peanuts and others) contain symbiotic bacteria called Rhizobia within root nodules of their root systems. These bacteria have the special ability of fixing nitrogen from atmospheric, molecular nitrogen into ammonia. Ammonia is then converted to another form, ammonium, usable by (some) plants.

This arrangement means that the root nodules are sources of nitrogen for legumes, making them relatively rich in plant proteins. All proteins contain nitrogenous amino acids. Nitrogen is therefore a necessary ingredient in the production of proteins. Hence, legumes are among the best sources of plant protein.