This refers to the seventh stanza of Longfellow’s A Psalm of Life:
Lives of great men all reminded us
We can make our lives sublime,
And, departing, leave behind us
Footprints on the sands of time.
Footprints on the sands of time are the things by which or for which – we are remembered by those who come after us. Napoleon wrote in a letter: ‘We should endeavour to do something so that we have not lived in vain, that we may leave some mark of ourselves on the sands of time.’
But not the mark we make when we sit down.