This is the 3540th most frequent Russian word.
рубль
The Russian currency (ruble).
'Рубль' is used here to denote a single unit of Russian currency.
У меня в кармане лежал один рубль.
I had one ruble in my pocket.
'Рубль' here is in the plural form 'рублях', indicating the use in a dative plural form within the sentence.
На рынке цены указаны в рублях.
At the market, prices are listed in rubles.
'Рубль' is used here in the singular genitive case 'рубля' to describe possession or relatedness to an attribute ('курс').