The marks left on the space of time are compared to the footprints that we leave on the sand. But those footprints are just temporary. So does the poet think that our effort could have a temporary effect?
The poet does not mean that, for sure, as that would go against the poem’s motive to show this life to be real and important enough. Here the point is to leave a mark or not to leave. The poet emphasizes on the need to leave a footprint on the eternity so that others can follow and be inspired by those. The word ‘sands’ is just symbolic of the vastness and eternity of time.