In the sonnet the speaker is talking about how the audience can see him approaching death. The first thing the speaker says that can be seen are the trademarks of Fall - the leaves changing and dropping and the weather turning cold. The second thing the speaker says that can be seen is twilight. Symbolically the twilight of someone's life is the time before they die. In him the audience can see the day fading and darkness approaching. The last thing the speaker says can be seen is a fire still glowing. He reflects on the fire of his youth which burned bright and strong but is not fading like glowing embers turn to ash.
