Latin for 'remember that you [have to] die', Memento mori is a philosophical reminder of death's inevitability.  Its roots go back to the Stoics and Philosophers of classical antiquity, c. 400 BC (Plato, Socrates, et al.)  Put simply, we mustn't take the simple things in life for granted.

A concept derived from the Buddhist teaching of the three marks of existence (三法印, sanbōin), specifically impermanence (無常, mujō), suffering (苦, ku) and emptiness (空, kū).