A Zen master might whack you with his stick for asking such a question, never mind what he’d do to us for trying to answer it. Nonetheless, here goes. Emptiness is the central insight of Buddhism, and what makes it unique among religions. According to Buddhism, neither we, nor other beings, nor any phenomenon in the universe, has a permanent, separate, and independent core, soul, or identity. Another way to look at it is interdependence: all relative phenomena are purely the product of external causes. As Thich Nhat Hanh says, the entire universe is present in a single flower, except for one thing—a self.
There are some traditional contemplations you can do to investigate this. Choose any object, say a chair, and see if you can find the one essential thing that makes it a chair. You can do the same thing with concepts themselves: is there an independent thing called “good,” or does its existence depend on “bad”? Another view of emptiness is that what things are empty of is our projections on them, from “chair” and “me” all the way up to “existence” and “nonexistence.” In the end, of course, emptiness has to go beyond intellectual understanding to direct experience. As the Tibetan yogi Milarepa sang, “Emptiness no longer intellect’s realm, what relief!” The Heart Sutra says, “When there is no obscuration of mind”—when we are no longer confused by our external projections and experience the wisdom of emptiness—“there is no fear.” That is why emptiness is so important—it is the antidote to suffering.