"Compare thee to a summer's day,
Thou art more lovely and more temperate"
In this metaphor, the beloved's beauty is compared to the pleasant and warm weather of a summer day. The speaker is saying that the beloved is more beautiful than anything in nature.
Another metaphor in the poem is the comparison between the beloved's eyes and the sun. Shakespeare writes:
"But thy eternal summer shall not fade
Nor lose possession of that fair thou ow'st;
Nor shall Death brag thou wand'rest in his shade,
When in eternal lines to time thou grow'st,
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee"
In this metaphor, the beloved's eyes are compared to the sun, which is a source of light and life. The speaker is saying that the beloved's beauty will never fade, just like the sun will never stop shining.