Imagine your friend has two cars. When he isn’t using one of them, the car is in his driveway. When you see your friend in town driving his car, you automatically are able to tell where his other car is: in his driveway. However you can only tell where the other car is when you observe either one of the cars. This is a fundamental property of entanglement. Entangled particles can only be described as a system and not independently of each other because observing them individually changes their properties.

Quantum entanglement is vaguely similar. When you have two particles with a correlated state, you can know what state the particle is by observing only one of the particles. Say particle 1 has a positive spin, you know by observing particle 1 that particle 2 has a negative spin. This is also applicable in computing, where if you know the state of one quantum gate, you can tell the state of another quantum gate when you observe it.

It’s important to note that particles are only in a correlated state as long as you don’t actively manipulate them to change their state. If you were to manually give particle 1 a negative spin then you wouldn’t be able to tell what state particle 2 is in.

Quantum entanglement has a lot of different possible applications in the real word. Things like quantum cryptography and quantum key distribution for example. The overarching concept is that by observing a quantum system you change its properties.