It's from Sonnet 18, and all it means is that sometimes the sun ("the eye of heaven") shines with too much heat.
Here's the passage:
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;
But thy eternal summer shall not fade...
So the sense is this: sometimes the sun shines too hot, often you can't see the sun's gold colour, and sometimes everything beautiful looks not-quite-as-beautiful, whether that's because of chance, or just natural change. BUT, the beauty of the beloved (her "eternal summer") will not change.
In this poem, the poet compares his girlfriend (or boyfriend) to the beauty of summer. The entire poem is about the temporary nature of beauty. On first reading it seems to be a lovely poem, in fact, after you read it a few times, it is a bit depressing. It focuses on the unavoidable loss of youth and beauty.
The poem says 'a summer day is beautiful but, today, you are MORE beautiful. But summer turns into winter, just like your young beauty will turn into ugly old age. Today you are more lovely than the sun, but you will fade, my love, and you will die, BUT my poem will not get old and everyone will remember your vanished beauty when they read my beautiful poem."
The line you ask about is in the first half. He is saying 'if I compare you to a summer's day, well, sometimes summer days are too hot, so you are better than a summer's day.'
And remember, 400 years ago they didn't have modern clothing, heating, or housing, tv etc etc. So winter was a horrible horrible thing, which in comparison made summer even more wonderful than modern people think it is. And yet, she is more beautiful than a summer's day... but it won't last. Summer's lease hath all too short a date.