This sonnet is about the inevitable passage of time.  In the octet (the first 8 lines of the sonnet) the speaker uses several images to convey the reality that everything that is young, alive, and vibrant, will fade away and eventually die.  He uses color imagery to make his point more clear. 

The opening line of the poem establishes the subject:  the clock to always moving forward.  From there the speaker seems to look around him for the evidence of the passing of time.  He notices that the bright sun of the day time fades to a "violet past prime."  He is describing that dark blue/purple sky just before the sky becomes black at night.  There is just a suggestion of light left.  The life of day has faded to the death of night.

The next line compares the sable curls of his hair (or someone's hair.)  Sable hair is a rich dark brown or black.  It the hair of a young person.  As that person ages, the hair gradually is taken over by (silver'd o're) by the white hair of old age.

The next three lines talk about the passage of the seasons.  He describes the green of summer when the trees provided a canopy, and contrasts that with the now barren trees that lack color and are instead are covered in a "white and bristly beard" of frost or snow. The season has passed from the life of summer to the dead of winter.

He makes his ultimate point in the final sx lines (sextet) of the the sonnet.  He states that everything that is beautiful must eventually die -- days, hair (a person), seasons. Nothing can stop this natural cycle, but having children will at least allow the natural beauty of a person to continue (not die completely) as that child will carry on the beauty of the present day person.