A metaphor is a <u>hidden</u> comparison between two things. Unlike a simile, a metaphor does not use words <em>like</em> and <em>as</em>.
The first line compares the sun to the 'eye of heaven'. But it does not say 'the sun is <em>like</em> the eye of heaven'. It just uses this phrase instead of the word 'sun', making the comparison implied.
Eye of Heaven. This is a metaphor that refers to the sun. The phrase "too hot the eye of heaven shines" means the heat of the sun is too much. The symbolism is relative to the sun being round with rays that look like an eye and heaven refers to something in the sky that you look up to.