In Sonnet 18, the speaker compares his beloved to a summer's day and then elaborates on how his beloved is superior to it. His first response to the comparison is to say of his beloved, "Thou art more lovely and more temperate." Temperate means moderate and less erratic. A summer's day (or summer itself), on the other hand can be too hot, overcast, etc.:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd;
And every fair from fair sometime declines,
By chance or nature's changing course untrimm'd,
From here, the speaker explains that his beloved's loveliness "outshines" or outlasts the summer loveliness. Here, loveliness can mean beauty, personality, spirit, etc. Summer and the seasons are subject to the change of time. When he says, "thy eternal summer shall not fade," and that Death can not shade her loveliness, the speaker implies that his beloved's loveliness is eternal. Here, the speaker denotes a metaphysical aspect to her loveliness as if, even in her death, the spirit of her loveliness lives on. But in the final lines of the sonnet, it is clear that the sonnet itself is how her beauty will live on, beyond the changing seasons and beyond her own death. "So long lives this, and this gives life to thee." "This" is the sonnet. In expressing strong feelings for his beloved, the speaker praises her eternal loveliness, immortalized in the sonnet itself.