[swift] What does "Fatal error: Unexpectedly found nil while unwrapping an Optional value" mean?

The errors EXC_BAD_INSTRUCTION and fatal error: unexpectedly found nil while implicitly unwrapping an Optional value appears the most when you have declared an @IBOutlet, but not connected to the storyboard.

You should also learn about how Optionals work, mentioned in other answers, but this is the only time that mostly appears to me.

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