This is the 13148th most frequent Russian word.
мертвых
"Мертвых" means "dead" (genitive plural).
Here, 'мертвых' is used as an adjective in the genitive case describing the plural noun 'о мертвых' (about the dead).
Книга о мертвых всегда есть в библиотеке.
The book about the dead is always in the library.
In this sentence, 'мертвых' functions as a noun in the plural form, referring to people who are deceased.
Они вспоминали мертвых на церемонии.
They remembered the dead at the ceremony.
Here, 'мертвых' is an adjective describing the plural noun 'деревьев' (trees), implying that the trees are no longer living.