This is the 3175th most frequent Russian word.
моего
"Of my" or "of mine" (genitive case, masculine/neuter singular).
The word 'моего' is in the genitive case modifying 'друга' (my friend).
Это книга моего друга.
This is my friend's book.
The word 'моего' is in the accusative case modifying 'брата' (my brother) as the object of the verb.
Я ищу моего брата.
I am looking for my brother.
The word 'моего' is in the genitive case modifying 'сына' (my son) indicating possession.