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 (1-4)
The first four lines make up the poem behind the poem. These four lines pack the basic point of the other 128 lines of the poem into a conveniently short space. To illustrate their point, imagine one of those weird toys—we don't know if they still exist—a kind of little piece of matter you drop into a bowl of water. Overnight it blows up into a bigger object—a much larger spongey dinosaur toy, say—inflated by the water. Well, seeing "a world in a grain of sand" is a bit like that—except the toy is any one of Blake's little metaphors listed in this poem, and the water is your mind. Yeah—the top of your head just flew off.
Basically, the idea is that any little thing in the world—a grain of sand, a wildflower—contains some sort of greater cosmic truth if you can look at it with enough energy and imagination. A wildflower is a miniature heaven, a grain of sand is a miniature world… and every person and other living thing, in Blake's view, is a miniature of the Divine Human or "Human Form Divine," which he identified with Jesus. Nearly all the metaphors Blake uses in the rest of the poem tend to be like worlds hidden in grains of sand. A robin in a cage is an example of freedom being crushed by tyranny in a universal way, for example.