The concept of quantum entanglement is that we are able to measure particles in a system based on one observation of parts of the system. So for example, say you have two particles that can either spin positively or negatively that form a system and you know that these particles are entangled and that the system produces a spin of 0. You observe one particle and it’s spin is negative. Automatically, you are able to infer that the other particle in the system is spinning positively.

This concept can be applied to a lot of “non quantum” things. For example, say your friend owns only two pair of shoes and that when he isn’t wearing one, the other pair is on his shoe rack. When you see your friend wearing his red shoes, you know where his blue shoes are. That’s because you know the rules of the your friends shoe system.

One misconception that people have about quantum entanglement is that changing the state of a particle/part of the system automatically changes the state of the whole system. That isn’t true. If you were to steal your friends shoes and wear them on your feet, someone seeing you wearing the shoes wouldn’t be able to tell where the blue shoes are because they are no longer entangled.

One other important concept in quantum entanglement is that the act of observing a system inherently changes the state of that system. If you don’t see your friend wearing his red or blue shoes, there’s no way for you to tell which pair is out and which pair is in. The moment you observe your friend and his shoes, the knowledge you have about the location of the other pair of shoes changes.

Applications of quantum entanglement are hard to explain. It’s present in concepts such as quantum cryptography and key distribution. You can create dice with quantum entanglement properties.

Quantum entanglement has all sorts of applications that make it very valuable in the real world. Especially in quantum computing but also that helps us observe systems and explain things in physics, biology, chemistry and many other fields.