One of the great things about being a kid is having the ability to imagine all sorts of things. You can imagine being able to fly, have super powers, be a movie star, be incredibly rich, travel to outer space, and so on.
Of course you can’t actually do some of these things, like have super powers. But some of them could happen to you. You might someday be rich, or famous, or an astronaut.
Unfortunately, even for the things you could do, you probably still have to wait until you’re older. And that’s what this poem is about; getting your fondest wish, but still having to wait. In the meantime, you can always continue to dream.