Memento Mori is Latin for reminder of death. Skulls which are represented in Northern European portraits and still lifes, and South European depictions of saints, of the 16th and 17th centuries are perhaps the most obvious examples of such subjects (see for example the anamorphic skull which is depicted in the foreground of Holbein's 'Ambassadors' and the precisely painted example in Steenwyck's 'An Allegory of the Vanities of Human Life').
Such elements are painted as a reminder that death is the great leveller, which puts an end to all worldly achievements. See entry for Vanitas.