millet vs food grain

millet and food grain both are nouns.

Word	Noun	Adjective	Verb	Adverb
millet	Yes	No	No	No
food grain	Yes	No	No	No

As nouns, food grain is a hypernym of millet; that is, food grain is a word with a broader meaning than millet:

millet: small seed of any of various annual cereal grasses especially Setaria italica
food grain: foodstuff prepared from the starchy grains of cereal grasses

Other hypernyms of millet include cereal, grain.

millet (noun)	food grain (noun)
small seed of any of various annual cereal grasses especially Setaria italica	foodstuff prepared from the starchy grains of cereal grasses
any of various small-grained annual cereal and forage grasses of the genera Panicum, Echinochloa, Setaria, Sorghum, and Eleusine