What is the Overmind? Funny you should ask: It's the question that has been plaguing readers of Childhood's End since 1953, and it will continue to do so until the book is out of print or humanity does, in fact, join the Overmind, in which case we suppose we'll figure it out then.
Truth is, we can't really tell you what the Overmind is because we don't really know. What we can provide you are a few thought primers, some pointers to help you collect your own thoughts on this alien race, so you might find your own answer to this question.
To Infinity and Beyond
Space is big. Unimaginably big. Astronomers measure its distances in light years, or the amount of distance light—moving at six hundred and seventy one million miles per hour in a vacuum—can travel in a year. The nearest star to Earth other than our sun is Proxima Centauri, roughly four-and-some-change light years away. So travelling at the speed of light, it would still take a human more than four years to make it to the nearest star. So when Karellen says, "The planets you may one day possess. But the stars are not for man" (14.35), well, we know he isn't playing.
And this is where the Overmind comes in. In the novel, the Overmind is mankind's ticket into the infinity of space. "Potentially infinite, beyond mortality" (23.28), this alien creature is freed from the physical bonds that hold us back in our present, fleshy form. The Overmind, then, represents endlessness.
In this way, the Overmind is almost like a wish fulfillment scenario. If mankind has the will, the urge to explore beyond his planet, then the Overmind can grant that wish. This fulfillment comes at a price, though: The Earth, the planet that has served as humanity's home but also its boundary from the reaches of space, must be destroyed. So as much as the Overmind symbolizes endlessness, it also symbolizes the end of Earth… and humans.
All for One
Ultimately, the Overmind is an alien race. As we mention in our "What's Up with Ending?" section, it shares many visual images with concepts like heaven and the Rapture, but in the end, it's still a physical being. What makes this being unique is that it's one giant mass of consciousness.
See, when we think of humanity, we think of a single entity, but that entity is broken up into various individual pieces: you, me, him, her, and so on. But the Overmind isn't like that. It's created by consuming many races across the galaxy, but once consumed, the individuals no longer seem to have any individuality. They are simply the Overmind.
George hints at this aspect of the alien when he thinks the following:
Only individuals can be lonely—only human beings. When the barriers were down at last, loneliness would vanish as personality faded. The countless raindrops would have merged into the ocean. (21.6)
The Overmind is the ocean, and humanity is the rain. Once the rain falls into the ocean, it's no longer the rain; it's just another part of the vast Pacific—or whatever ocean you prefer—and so, too, for humanity once it joins the Overmind.