This is the 644th most frequent Russian word.
наш
"Наш" means "our" or "ours" in English.
The word 'наш' is a possessive pronoun in the nominative case, used here to indicate ownership or possession of the noun 'дом' (house) by the speaker and their group.
Это наш дом.
This is our house.
The word 'нашего' is the masculine singular form of 'наш' in the genitive or accusative case, used here because the noun 'друга' (friend) is the direct object of the verb 'встретили' (met).
Мы встретили нашего друга.
We met our friend.
The word 'наши' is the plural form of 'наш', used here to indicate possession of the plural noun 'стулья' (chairs) by the speaker and their group.