It is Latin for "remember you will die" and is "carpe diem" ("seize the day") more melancholic brother. Anecdotally whispered to Roman emperors to remind them that despite all their glory, their victories, their power, they were still mortals. It is more a warning against hubris than a call to live in the moment.
Found this monument on Edinburghs Grey Friars Cemetery.
Steepshot