To him who in the love of Nature holds Communion with her visible forms, she speaks (1-2)
This is probably the kind of opening that doesn’t make a lot of immediate sense, and maybe just seems like old-timey poetry mumbo-jumbo. We think it’s worth digging into, but it does need some context. To put it simply, Romantic poets like William Wordsworth (whose work had a big influence on this poem) believed that nature had healing power. They thought that by going outside, and also by reading poetry, you could join in a kind of spiritual union ("communion") with objects ("visible forms") in the natural world. So, this line means that, if you really love nature, she will speak to you, as if she had a voice. There’s plenty more in there, but that’s the general idea.
She has a voice of gladness, and a smile And eloquence of beauty, (4-5)
Here the idea is that nature ("She") talks to us in different ways, depending on our mood (that’s what he means by "various language" in line 3). When we are happy, her voice is happy, full of "gladness." It’s as if nature was smiling at us, and the beauty we see around us is like a beautiful speech. We know you’ve had this feeling, when you’re having a great day, and it seems almost like the birds and the trees and the sun are smiling with you.
Go forth, under the open sky, and list To Nature’s teachings (14-15)
Again, this is a pretty classic statement for a Romantic poet. Go outside. Listen ("list") to nature. Everything you need to know to feel comforted and happy is out there. She’s the only teacher you’re ever going to need. Don’t sit inside worrying about death. Go out and join with the world, feel its beauty, let it make you whole. OK, now maybe we sound a little bit like we’re starting a cult. We promise we’re not. (Shmoop…Shmoop…the Shmoop will set you free…)