This is the 17117th most frequent Russian word.
стадиона
"Стадиона" is the genitive or accusative case form of "стадион," meaning "stadium" in English.
In this sentence, 'стадиона' is the genitive case of 'стадион', used after the preposition 'мимо' to indicate passing by.
Я иду мимо стадиона.
I am walking past the stadium.
Here, 'стадиона' is again in genitive case, but it indicates possession, part of the structure viewed.
Мы видели крышу стадиона из нашей квартиры.
We saw the roof of the stadium from our apartment.
In this sentence, 'стадиона' is in genitive case, contributing to a genitive construction specifying a section or component of the stadium.