This is the 15287th most frequent Russian word.
посольстве
"Посольстве" means "embassy" in English.
In this sentence, 'посольстве' is used in the prepositional case to indicate the location where the action took place, 'в российском посольстве' translating to 'at the Russian embassy'.
Я приобрел визу в российском посольстве.
I got my visa at the Russian embassy.
Here, 'посольства' is the plural nominative case of 'посольство', referring to embassies in general and their role in international affairs.
Посольства играют важную роль в международной политике.
Embassies play an important role in international politics.
In this sentence, 'в посольстве' uses the prepositional case, indicating the place where the person worked, 'at the embassy'.