In this sonnet, the speaker describes himself as aging and getting closer to death. In the first four lines, he suggests that his beloved might look at him and think of the late fall, when only a few yellow leaves remain on the trees, when the birds sing no more because of the cold. Winter, then, is symbolic of his impending death.
In the second quatrain (set of four lines), the speaker suggests that his beloved might look at him and be reminded of "twilight," when the sun has already set but its light is not yet quite gone. He calls night "Death's second self," emphasizing that he is old and close to death, metaphorically referred to as a kind of night.
In the third quatrain, the speaker says that his beloved might look on him and be reminded of a fire that "on the ashes of his youth doth lie." Literally, the wood we use to start a fire burns and turns to ash, and any new wood must be placed atop those first ashes. Figuratively, the speaker means that his youth is all gone, completely consumed by time, as the ashes are now consumed by the fire they helped to build. In this way, the ashes are metaphorically linked to his youth, which is now spent and burned away. It is another metaphor for his advanced age and nearness to death.