KISS is an acronym for "Keep it simple, stupid". The principle stands for keeping systems easy to use and not to add complexity.
The KISS Principle was coined by Kelly Johnson, an engineer on the Lockheed Martin's Skunk Works, which was an advanced aircraft development program. Kelly had put forth a challenge to his engineering team that the jet they were designing should be easily fixed by an average mechanic with a few tools during combat conditions. The KISS Principle started out in the US Air Force but has taken root in the software development world.
Simple does not mean plain. On the contrary, it means to remove complexity and to keep all the important information. In other words, making a system user-friendly. This is not an easy task. It takes thought, time and dedication at every stage of the project to keep things simple and to keep complexity from slowly creeping in.
Next time you design a website or mobile app, remember to keep asking yourself if you are adding unnecessary difficulty or keeping things simple.