Everything you need to know about the OLED display rumored to be on the iPhone 12

In a normal year, Apple shows off its new line of iPhones in September. 2020 has been anything but normal, so naturally we didn't get that. However, new iPhones are coming and some of the rumored new features require some explanation.
We don't fully know what to expect from the iPhone this year, but one of the most interesting rumors suggests Apple will launch four new models, all with OLED displays. 
Wait, what kind of displays?
If you haven't been keeping up with the latest advancements i