There are two main themes in Sonnet 73. Both are universal ideas, but the first—that of the mortality the speaker is aware of in himself as he ages—is something all of us can appreciate and relate to as we age. The second, meanwhile, addressed in the closing couplet, is concerned with the love of a younger person for an older one—a concept which, while certainly not parochial or limited in scope, may not be something everyone can understand.
The speaker observes that he himself is in the autumn of his lifetime. He uses natural imagery to suggest that he, at this point in his life, resembles a tree bedecked with autumn leaves, standing in a "twilight" which will give way soon enough to the darkness of death. His youth lies now in "ashes," and this must surely be evident to anyone who sees the speaker. The speaker's message in the poem to his beloved, then, is that anyone who can still love someone whose youth is far behind them must love very strongly indeed. That kind of love is "more strong" because it is not a fickle love compelled by youth and because it knows that the beloved must soon depart.