I have provided a useful link below that gives a detailed summary, plus some background information that should help a lot. But in general, the poem is a commentary on the unity that exists between human beings and nature, and has a detailed commentary on how we should not fear death, because we will just become one with nature, which is the best of fates. It starts off with talking about how nature can increase a person's joy or sorrow, and if one was feeling down, they should walk in nature to lift their spirits. Nature becomes the narrator for the duration of the poem, and tells humans to not fear death, but instead to be comforted that when we die we become one with nature, the most noble of creations, and become one with everyone else that has passed on also.
I hope that helps, and make sure to check out the links below, as they are very helpful also.