Entanglement is simply the idea that quantum processes are correlated. Therefore you can infer things like intrusion more or less instantly.
A good example I’ve heard is that it’s like having a pair of black socks. You realize you accidentally put on only one of them, and instantly know that the other sock must be back at home.
But the socks didn’t talk to each other or anything. No fundamental communication between two parties can occur. (Certainly no “teleportation” in the way we think of it occurred.) You can see by deconstructing a little bit. How do we coordinate, for example, which socks are left behind and which are worn and such? By means subject to the speed of light such as radio signals.
If we tried to instead orchestrate a scheme around this ahead of time, we’d quickly run into a problem. In quantum mechanics, I can’t say at all for sure whether you left behind the black sock, the red sock, the blue sock, etc. All I can is that a sock was left behind.
So what’s the point, you might bitterly complain. Sure, we can’t say, for example, “If a black sock was left behind, then the letter A. If a blue sock was left behind, then the letter B…” and so on. We’d have no idea what we would get. Any attempted message would be completely garbled.
But the advantage of such quantum communication is due to possible interference. I see that I have a black sock, but I don’t know that just because you saw the other one that someone hasn’t just swapped in an identical one.
And it turns out that this scenario is actually very, very improbable in real-life cases.
The really random, exotic rules of quantum mechanics mean that, if you tried to interfere (by making a measurement as an interloper), you would basically be rolling the dice as to whether you caused the end-party to see the black sock they were expecting instead of a red sock, a blue sock, whatever. If it wasn’t the black you were expecting, then you’d know someone was being a creep.
Now, take this same idea, and multiply it across, say, 256 bits of data that compose an encryption key.
To avoid being detected, someone would have to roll “black” out of often many many possible outcomes (not just red or blue or whatever) every single time out of 256 in this example just to avoid detection. That’s mathematically extremely unlikely. And if we do detect them, we basically just change the key very quickly and dare them to try again.
That’s pretty hard to crack. That improbability of detection is why we like quantum entanglement on a practical and technological level.