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 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.
Он гордился достижением столь сильного друга.
He was proud of the accomplishments of such a strong friend.
Here, "сильного" is in the accusative case, describing and modifying "спортсмена."