First, a little bit of background information.

If we look at a car traveling down the road, we know exactly where it is, and how fast it's going, and it doesn't matter if we are looking at the car or not, it just has a certain position and speed. But in quantum mechanics, there are properties of objects that aren't well defined until we look at the object, which is called taking a measurement.

For example, let's say we draw an arrow on an electron, which is one of the building blocks of all the stuff around us. Let's say that arrow could only be pointing up or down. According to quantum mechanics, we can set it up so that the arrow isn't actually pointing up or down until we take a look to see which direction it's pointing. Scientists say that the arrow is in a superposition - it's pointing both up and down at the same time. Let's say we measure the direction of the arrow, and it's pointing up. After that, it will definitely be pointing up. But before that, it could have been pointing in either direction. It took the act of measuring the direction to actually force the arrow to be pointing in one of the directions. It's kind of a weird concept, but quantum mechanics has a lot of weird concepts in it.

Quantum entanglement is when we take two objects, and make it so that they both have the same "quantum superposition". It's similar to the example above, but now let's say we have two electrons, and we've "entangled" them somehow. Before we measure anything, the arrows will still be in a superposition of up and down - there's no way to say that it's pointing in either direction. But once we measure the direction of the arrow of one particle, we'll know what the direction of the other particle's arrow is. Since they were entangled, they'll have the same properties, and knowing information about one particle will tell us information about the other particle, even if we haven't measured it yet.

One way to generate entangled particles would be by using something that is essentially like a fancy prism. One important thing about light is that scientists have found out that light is actually made up of particles, called photons. A photon is like the smallest amount of light we could have. If we have a normal prism, we can split regular sunlight into a rainbow. But if we use a very specialized prism, and try to send one photon through it, it's possible to get out two photons that are entangled together.

One application of quantum entanglement may be quantum computing. A quantum computer would work kind of like a normal computer, but it would use very, very small objects, like electrons and photons, to do calculations, instead of electricity. Entangled particles would mean we could do the same calculation more than once, which would help check for any errors. Also, quantum entanglement can be very useful for sending secret codes. We could make a secret code using some of the properties of electrons or photons. Then we could create a new set of entangled electrons or photons to send to someone else. The other person can be sure that the information hasn't been looked at by someone else if the particle is still in the "superposition" state I mentioned above, since if someone came and intercepted the message, that measurement would ruin the entangled state.