This is the 3569th most frequent Russian word.
сильного
"Сильного" means "strong" or "powerful" (genitive or accusative singular masculine/neuter form).
Here, "сильного" is in the accusative case, describing and modifying "спортсмена."
Я встретил сильного спортсмена на соревнованиях.
I met a strong athlete at the competition.
Here, "сильного" is in the genitive case, describing the hero as being strong.
У меня есть книга о сильного героя.
I have a book about a strong hero.
In this sentence, "сильного" is again in the genitive case, used to specify the number or characteristics of the friend.