This is the 16666th most frequent Russian word.
сидений
"Сидений" means "of seats" (genitive plural form).
Here, 'сидений' refers to physical seats in the bus, used in the plural genitive case to express absence.
В автобусе были места без сидений.
In the bus, there were places without seats.
In this sentence, 'сидений' again is in the plural genitive case, this time showing possession or relationship (the color of the seats).
Ты заметил, что цвет сидений изменился?
Did you notice that the color of the seats changed?
Here, 'сидений' is utilized in the plural genitive case to indicate an object of prepositional modification (covers for the car seats).