To see a World in a Grain of Sand And a Heaven in a Wild Flower, Hold Infinity in the palm of your hand And Eternity in an hour.
Blake's speaker gives us a handy key to the rest of the poem in these first four lines. In a nutshell—or, in Blake's own words, "in a grain of sand"—it provides the greater theme and message of the poem, compressed down to bite-size: a crunchy wisdom nugget. They're cryptic, mystical lines.
He is suggesting that the entire universe can be a contained in a grain of sand in a spiritual way—it's not that you would literally see the entire world by looking, physically, at a grain of sand or something. It's more that a glimpse of something tiny can provoke you to imagine something really big—tons of images can come pouring in. According to Blake, that's the whole point of a metaphor. That's what it does.
A wildflower can contain a heaven because the beauty of a flower is (for Blake) a piece of heaven, and it can make you imagine or visualize that better and more beautiful world. Also, a grain of sand is a world in miniature, because it's a tiny piece of a world, of earth. The little parts reflect the big things that they're parts of.
The speaker keeps playing with this idea of big things contained in small things. Fitting infinity in the palm of your hand seems like a contradiction—since infinity is endless and the palm of your hand isn't. The same thing goes for fitting eternity into an hour. But the speaker doesn't think infinity is endless space or eternity is endless time. He thinks they're part of the "Eternal Now," so to speak—you really enter eternity and infinity if you get into a state of poetic imagination in the present moment. (It's like The Daily Show's"Moment of Zen"—kind of.) Blake's compadre Emily Dickinson meant the same thing when she wrote, "Eternity is comprised of Nows." Far out, right?
Unlike the rest of the poem, these first four lines are different. They're written in balladmeter (four beats in one line, then three in the next)—like "Amazing Grace," the Gilligan's Island theme song, and a billion Emily Dickinson poems. They're in a quatrain, too—whereas everything else is a couplet in iambic tetrameter (like iambic pentameter, but with four feet instead of five). Don't worry about this alphabet soup of terms, though. For all the explanations, see the "Form and Meter" section.