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.