This is the 12425th most frequent Russian word.
квартирах
"Квартирах" means "apartments" (prepositional case, plural) in English.
The word 'квартирах' is in the prepositional case, used to refer to a location ('in the apartments').
В этих квартирах всегда очень уютно.
These apartments are always very cozy.
Here, 'квартирах' is in the prepositional case, used to specify 'in which' location an event is happening.
Я заметил, что в двух квартирах горит свет.
I noticed that lights are on in two apartments.
The usage of 'квартирах' here indicates the prepositional case, specifying a relative position ('on the apartments').