Shakespeare is drawing a comparison between the subject of the sonnet and a summer's day, claiming that the person is "more lovely and more temperate." While summer is beautiful, it lacks the balance and the enduring beauty of the subject of the sonnet. Summer begins with "rough winds that shake the darling buds of May" and comes to an end, having "all too short a date." Sometimes it is too hot, and sometimes it is cloudy. But the subject of the sonnet, and here it is important to understand that Shakespeare is talking about the person as memorialized in the sonnet, will remain beautiful forever:
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou growest:
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
The sonnet has made its subject immortal, or more accurately, beautiful as long as the sonnet is read by people. Realistically, of course, Shakespeare is acknowledging that the actual life of the person is like the summer, too short and often not without its imperfections. But in poetry, the subject achieves a timelessness unattainable in life.