This is the 13311th most frequent Russian word.
мужчины
Men.
In this sentence, 'мужчины' is used as the plural nominative form of 'мужчина' (man) to refer to men in general.
Мужчины часто увлекаются спортом.
Men often take interest in sports.
Here, 'мужчин' is the plural genitive form of 'мужчина', used as an object of the verb 'видел' (saw).
Я видел группу мужчин в парке.
I saw a group of men in the park.
In this case, 'мужчин' is the plural genitive form of 'мужчина', used in the prepositional phrase indicating origin.