ELI5: Quantum entanglement

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.