The surface meaning of the poem is about a person, presumably a man, who is in love with this other person, presumably a girl.  Obviously, the speaker has strong feelings towards the subject of the poem.  The speaker compares the subject of the poem to the natural conditions of weather and then a brief comparison to death.  However, the speaker believes that the subject of the poem is more beautiful, more consistent, and more radiantly powerful than either.  The symbolic meaning/ theme of the poem reflects a deep love the speaker has for the subject, almost to say that it is the type of affection that transcends natural conditions such as meteorology or death.  It is the type of true love and reverence that anyone who "can breath" or "can see" would understand.  The poet employs several devices to assist in evoking this devotion.  The first would be his use of the metaphor in comparing the subject of the poem to weather.  The opening line heralds in this link: "Shall I compare thee to a summer's day?"  In this first line, we see the metaphor introduced, and the poet will continue and expand this comparison throughout the poem.  The poet employs several examples of "figurative language" in lines 2-4 with the imagery of "temperate" and "darling buds," which create a mental picture of natural beauty in one's mind.  This natural picture is complemented with the subject of the poem when he says in line 2, "Thou art," which means that the subject is more lovely than such images.  In the second stanza of the poem, the speaker explains how weather can represent some discomfort, such as line 5 with "too hot the eye of heaven," meaning that it is too hot during summer at times and the clouds that build in line 6 ("And often is his gold complexion dimm'd).  Finally, in the last two lines, the speaker talks about how seasons change.  He employs these examples of figurative language and imagery to bring out in the third stanza how the subject of the poem is so radiant that the subject represents more consistency and strength in beauty than the weather.  The speaker claims that "thy eternal summer shall not fade."  (Line 9).  In this, one can see the enhancement of the nature metaphor, for death is a part of nature and fading flowers are essential to the growth cycle.  However, this apparently will not happen to the subject of the poem.  In lines 11-12, the speaker goes as far to suggest that the subject will outlast death in their beauty and radiance.  Age, indeed, will not wither her.

The closing couplet brings to light the universality of this beauty.  This is significant because the couplet reflects that any individual would share the same feelings that the speaker has "so long as men can breathe, or eyes can see."  This is effective because it highlights the theme of the poem.  It makes a subjective, or personal, emotion, a universal one, applicable to everyone.  Thus, the title of the sonnet, "To his love," is now something that we can say to everyone, "To our loves," if you will.