Could you explain to me what Ralph Emerson is trying to say in his essay "The Poet"?

First, you need to understand that Emerson is a transcendentalist.  In this context, that means that he believes that human beings need to seek "an original relation to the universe," meaning that they need to see how they are a part of the universe and the universe is a part of them.  In this way, people need to get outside themselves and imagine the world/universe in new ways.

To Emerson, this is what a poet does and that's the main point of "The Poet."  In the essay, he argues that poets make people free.  They do this by using language in new ways so as to get people to look at things in new ways.

It's a long essay, but that is a very brief statement of what it's about...