Diodes are one of the most important parts of any electronics project. If you don't understand what they do, you'll destroy any electronics project you work on. Thankfully, Adafruit has a video that explains how they work.
Diodes protect your circuits by forcing the electrical current to go in a specific direction. Knowing when and where to put them means you won't accidentally short circuit the whole system. Adafruit's video is meant to help kid understand how diodes work, but that doesn't mean we can't all learn a little something from it.
D Is for Diode [YouTube]