For a true ELI5, this will require leaving out a lot. This is simply an analogy. Analogies don’t hold up perfectly with very complicated things, so be careful about using them in place of complete understanding or learning more.

The basic principle is having two special particles. These particles are kind of like twins. Created together, they become “entangled” and share a special bond. It’s not magic but more like a connection that is hard for others to understand and see.

Now, let’s say we take Particle A and put it in a box, and Particle B in another box. We take the boxes far away from each other, even on different sides of the world.

Whatever happens to Particle A will instantly affect Particle B, no matter how far apart they are! It’s like they can still talk to each other and know what the other is doing.

Another way to think of it is like having two magic coins. If you flip one coin and it lands on heads, the other coin will always land on tails, no matter how far apart they are.

Scientists are still trying to understand exactly how this happens, but it’s a very special and strange thing in the world of of very small things. Some think it shows that there is a way that very small parts of the stuff in the universe are connected that we cannot measure yet, or that many different possibilities exist and we only see one of them when we look for it.

If we could create this connection reliably and stably, we could potentially use it send information across distances nearly instantaneously! After all, a lot of the information we send right now is just 1s and 0s which we put together to make more complicated messages. This has uses in protecting information to keep it secret, making very fast computers, and maybe even “teleportation” by creating a duplicate at the other end of the connection, to name a few.

Like many fields of science, we are learning more about quantum stuff all the time, so this could change really fast. If you’re interested in learning more about quantum theory and research you’ll need a strong background in math and science. Algebra, trigonometry and classical physics would be a good first step (of many).

Okay, I am up for a challenge…

I am going to skip over all of the complicated Quantum Theory stuff and give you some general rules via a metaphor.

Let us say I have a box, in the box are two balls. Due to the laws of nature, the only way to be able to have two balls in this box is of one of them is red and the other is blue. If they are both red or both blue, they cannot go into the box.

Now, if I remove one of the balls without looking at it, carry it far away (as far away as you like…light years…other galaxies…etc) if you look at that ball and it is blue, the one you left behind has to be red because they were together in the box.

This cannot be used for instantaneous communication because you cannot influence which one is red or blue.

Part of the bizarre nature of this phenomenon is that the balls are neither red nor blue before one is observed. This is why it is called “spooky action at a distance”. For the same reason Schrodinger’s cat is both dead and alive in the box.

As for applications, it is debatable. Because the information is destroyed if observed at the wrong time (ie. wiretapped), there are discussions about using it to transmit encryption keys. Tests are ongoing.