This is the 11026th most frequent Russian word.
географии
"Geography."
In this sentence, 'географии' is in a genitive case, indicating possession or relation, here of 'different countries'.
Я увлекаюсь изучением географии разных стран.
I am passionate about studying the geography of different countries.
Here, 'географии' is in a dative form used in the phrase 'по географии', meaning 'about geography'.
Учебник по географии очень интересный и подробный.
The geography textbook is very interesting and detailed.
In this case, 'географии' is in a prepositional case, used after the preposition 'о', meaning 'about'.