This is the 7064th most frequent Russian word.
белорус
"Белорус" means "Belarusian" (a male person from Belarus).
Here, 'белорус' is used as a noun to indicate a person from Belarus, describing their nationality.
Он белорус и гордится своим происхождением.
He is Belarusian and proud of his heritage.
'Белорусская' is the adjectival form of 'белорус', used here to describe 'кухня' (cuisine) as originating from Belarus.
Белорусская кухня славится своими вкусными блюдами из картофеля.
Belarusian cuisine is known for its delicious potato dishes.
'белоруса' is the genitive singular masculine form of 'белорус', used in the context of meeting someone described by this nationality.