Remember that scene in War Games when the socially broken code monkeys were explaining to aspiring hacker Matthew Broderick all about "back doors" (i.e. secret pathways planted by programmers)? Well, that's actually a thing.
Coders have a storied tradition of baking in secret passageways (or sometimes, just fun little Easter eggs) that can only be accessed by inputting a special "key." And so that tradition continues in the mobile age.
Unstructured Supplementary Service Data (USSD) -- sometimes known as "quick codes" or "feature codes" -- is an extra-UI protocol, which allows people to access hidden features. This protocol was originally created for GSM phones, but can be found on CDMA devices as well (if that's a bunch of acronym gibberish to you, here's a quick primer).
These publicly available backchannels allow users to directly communicate with their service provider's computers and/or access back-end features in their device. They are accessed by inputting them into the phone's dialer (the screen you use to start a phone call) and usually begin and end with the * or # keys with a sequence of numbers in between (there's close-to-zero chance that anyone would accidentally access them).
They're not terribly practical. Most people don't really need to know how their local cell towers are performing or what their IMEI number is (more on that later). Still, it can be fun to play around and see what unexpected functionality your phone is hiding beneath the surface.
We would love to provide you with a comprehensive list of the dozens of codes out there, but that would be an exercise in futility. These codes seldom work across different carriers, OSes or phone models (or even on generations of the same model).
If you really want to try them out, your best bet may be to Google your phone's make and carrier + "USSD" for a tailored, comprehensive list. I attempted a number of codes using an iPhone SE (while trading out numerous carrier SIM cards) in addition to a Galaxy S5 and Galaxy S7 Edge running on AT&T. Some of them worked! Check out the list below for 13 codes that I can confirm worked on at least one device. Good luck and have fun!