This sonnet by Shakespeare addresses the soul of the speaker, which, according to the speaker, is trying desperately to invest in this life and the here-and-now, even though this is futile, due to the eventual death that awaits us all. The soul is personified as the owner of a property that is spending lots of money on its "fading mansion," even though, as the rhetorical questions that follow this description show, the soul is fighting a losing battle in investing all of its attention on such temporal activities:
Shall worms, inheritors of this excess,
Eat up thy charge? is this thy body's end?
The speaker, based on these unquestionable realities, therefore tries to reason with his soul, advising him to focus not on outer appearances but inner beauty, and counseling him to "Buy terms divine in selling hours of dross." How the soul spends its time and lives its life enables it to get the better of death, as the final rhyming couplet clearly displays:
So shalt thou feed on Death, that feeds on men,
And Death once dead, there's no more dying then.
The soul can metaphorically "feed" on death if it focuses on things eternal rather than temporary things, therefore killing death and entering a state where there will be no more dying. This poem is above all about eternal realities and how to live our lives in response to them.