This is the 610th most frequent Russian word.
мужчина
Man; male person.
The word 'мужчина' is used here as a noun to refer to an adult male individual in context of his profession.
Этот мужчина работает врачом.
This man works as a doctor.
The word 'мужчина' is used here in the accusative case to describe an adult male who is performing an action (helping children).
Я видел мужчину, который помогал детям.
I saw a man who was helping children.
The word 'мужчина' is used in the genitive case here to describe possession.