In this poem, the author is giving credence to the idea that a poem is eternal and unchanging (because it is written, and can last forever as a result) and also mocking the fact that people expect the meaning to change or have some power over their lives. He is mocking, a bit, people who come to poems to expect complete and total profundity, or a life-changing experience. He states that it won't do that-it's just a poem. You are constantly changing, poem or not:
"You may come into it briefly,
But no one will find you here, no one.
You will have changed before the poem will."
So, a lot of the poem centers on that theme of poetry's inefficacy, or lack of real power to change. It is not the poem that is beautiful, it is life, and that the beauty we each possess "has no place here," in a poem. He comments that "Night is the sky over this poem," saying that nature, life, people, and living is what is real and true, not this poem that is after all, just "empty...black...most beautiful in its erasures." His poem symbolizes life's beauty and meaning, and poetry's inability to capture that beauty.
I hope that helps a bit! It's an interesting poem, ironically being profound while trying not to, making you think about it even as it forbids you to do so. So, good luck!