What this means in practical terms (if anything about quantum mechanics can be considered practical, that is) is that if we know something about one particle, we know something about its entangled mate. But it's weirder than that. Say you have a pair of gloves. If you know that one glove is right-handed, you automatically know that the other is left-handed, even if it's billions of light years away. But with entangled particles, the act of measuring one particle actually changes the state of the other. It's as if both gloves were in a superposition of right- and left-handedness—that is, both right- and left-handed at the same time—and only when you observed one did it become right-handed, thereby making the other become left-handed that very instant. That is, even if you check both gloves at the same microsecond from opposite sides of the universe, one becomes right-handed and the other becomes left-handed. That's why Albert Einstein called quantum entanglement "spooky action at a distance."
But wouldn't this break the universal speed limit known as the speed of light? No, and here's why: the information isn't "sent" in an instant, the way you might send an email. The relationship between the particles already exists from the time the particles first interacted and became entangled. As Frank Wilczek writes in Quanta Magazine, "in all known cases the correlations between an [entangled] pair must be imprinted when its members are close together, though of course they can survive subsequent separation, as though they had memories." The particles are "sending" information without sending anything at all. If you think that sounds like a fantastic way to create a supercomputer, you're onto something—it's precisely why quantum computing has such a huge potential.