This is the 16433rd most frequent Russian word.
рубли
Rubles; the currency of Russia.
In this sentence, 'рублей' is used in the genitive plural form to express a quantity in the context of purchasing something. It refers to the Russian currency, rubles.
Я купил книгу за двести рублей.
I bought a book for 200 rubles.
Here, 'рубли' is used in the nominative plural form to indicate the subject of the sentence, which are physical currency notes or coins.
Рубли лежат на столе.
The rubles are lying on the table.
In this example, 'рублей' is used again in the genitive plural form but in a negative construction to indicate the absence of funds in terms of rubles.