。
I pray for their peaceful rest in the afterlife.
Literal
Honorable [gen-の] afterlife-happiness [obj-を] honorific-pray-for-[polite-します].
ご冥福をお祈りします is the stock polite condolence phrase used at funerals and when hearing of someone's death — the Japanese equivalent of 'rest in peace / my condolences.' 冥福 literally 'dark fortune' refers to happiness in the afterlife (a Buddhist concept). The pattern ご+noun+お+verb+します layers two honorific markers. A formulaic, ritualized expression — essential funeral-context vocabulary.