In Shakespeare’s 18th sonnet, the speaker is almost unquestionably a woman’s lover. The man, speaking to the one he loves, is speaking to her about the fact that she is beautiful. It isn’t clear whether these lovers are fictitious; perhaps the male speaker is supposed to be Shakespeare himself, almost as an autobiography. While we don’t know the truth of who the lovers really are, it seems more probable that it is Shakespeare speaking as himself. Ordinarily people don’t just concoct love poems with no object of it [their affections] in mind when they write; it doesn’t seem probable that such impassioned words, even from Shakespeare, would have just been an exercise in rhyming and pretty talk. That everything is made up in the sonnets is possible—just unlikely.
The overall message of the poem is a simple one: If you were to paraphrase it, you might say, “Your beauty exceeds that which is natural.” The words obviously draw an association between the lady and nature, and her admirer/lover tells her that she reminds him of nature—especially as experienced on a warm summer day, to be precise—though he may just be using creative license when drawing his comparison, however. The sonnet doesn’t have a metaphor per se; the closest we get is the opening line where the man tells the woman that her beauty could be compared to that of a summer’s day. There are similes, however, such as that of an “eternal summer.”