This is the 2843rd most frequent Russian word.
автобусы
Buses.
Here, 'автобусы' is used as the subject of the sentence in the nominative case.
Автобусы приезжают на остановку каждые десять минут.
Buses arrive at the stop every ten minutes.
In this sentence, 'автобусы' is in the accusative plural form as the object of the action 'saw'.
В городе я видел новые автобусы.
In the city, I saw new buses.
Here, 'автобусов' (a form of 'автобусы') is used in the genitive plural to indicate absence in the sentence.