This is the 17675th most frequent Polish word.
zmarłych
"Zmarłych" means "of the dead" in English.
The word 'zmarłych' is used as a plural genitive form accompanying 'groby' to specify 'graves of the deceased'.
W dniu Wszystkich Świętych odwiedzamy groby zmarłych.
On All Saints' Day, we visit the graves of the deceased.
Here, 'zmarłych' functions as an attribute modifying 'dusze', indicating 'souls of the deceased'.
Modlimy się za spokój dusz zmarłych.
We pray for the peace of the souls of the deceased.
In this sentence, 'zmarłych' is again in the plural genitive form, modifying 'opowieści', expressing 'stories about the deceased'.