When a reader analyzes diction in a poem they are paying special attention to the word choices made by the poet. The reader should give thoughtful consideration to the denotation (dictionary definition) and the connotation (thoughts, feelings, associations) of specific words in the text. The reader is trying to discern why the poet used the words that he did. Here are examples of diction -- one from each section of the sonnet.
In Sonnet 73, Shakespeare has a speaker talking about how he is aging. Each of the three quatrains use a metaphor of aging to help make his point. In the first quatrain, the poet uses the metaphor of life being like a calendar year -- with old age coming the fall and death coming in winter. Within this quatrain, he uses the word "choirs" to describe the now empty branches of a tree without leaves and without birds. The word evokes the beauty of a full choir singing and leaves the reader to imagine how quiet and strange this is now.
The second quatrain compares life to a single day, with old age coming in the evening, and death coming at night. Here he uses the word "seals." Seals suggests lots of ideas that relate to death and sleep: the seals are sealed shut, the coffin is sealed closed, a will can be sealed and made official.
In the third quatrain he compares life to a fire, with old age being the last vestiges of the fire as it is being smothered by it own ashes. He uses the word "consumed" to connotatively suggest that the fire eats the wood/fuel that gives it life or "nourishes" it as the end of the line says.