There's no way to fully summarize this poem, because there is so much in the poem. Seriously – Walt Whitman changes topics almost every other line. But there are a few main ideas you should know about before starting.
Let's start off with the basics: our speaker, who is actually named Walt Whitman, declares that he's going to celebrate himself in this poem. He then invites his soul to hang out and stare at a blade of grass. It's the party of the year.
He explains how much he loves the world, especially nature, and how everything fits together just as it should. Everything is good to him, and nothing is bad that doesn't contribute to some larger good. Nature has patterns that fit together like a well-built house.
The speaker divides his personality into at least three parts:
Whitman thinks it's important for people to learn through experience and not through books or teachers. (If you like this idea, check out his short and sweet poem called "When I Heard the Learn'd Astronomer" https://www.shmoop.com/learnd-astronomer/; you'll be a fan.)
A child asks him what the grass is, and he doesn't have an answer, which gets him thinking about all kinds of things, but especially about all the people buried in the earth who came before him. He identifies with everyone and everything in the universe, including the dead. He imagines that he's a bunch of different people, from a woman staring at naked bathers to a crewman on a ship during a naval battle. His soul takes him on a journey around the world and all over America.
Whitman tells us a bit about what he believes and what he's opposed to. Let's start with what he's opposed to:
On the flip side, Whitman believes that:
At the end of the poem, he says that he's going to give his body back to nature and to continue his great journey. He'll be hanging out ahead on the road, waiting for us to catch up with him.