This is the 10263rd most frequent Russian word.
боксеров
Boxers (plural form).
'боксеров' is the accusative plural of the masculine animate noun 'боксёры', used here as the direct object of the verb 'знаю' (I know).
Я знаю боксеров, тренирующихся в этом зале.
I know boxers who train in this gym.
'боксеров' is the genitive plural of the masculine animate noun 'боксёры', used here to indicate possession or association ('life of...').
Жизнь профессиональных боксеров часто бывает трудной.
The life of professional boxers is often difficult.
'боксеров' is the genitive plural of the masculine animate noun 'боксёры', used here to describe the group for whom the competitions are organized ('for the boxers').