KISS is an acronym that stands for "Keep It Simple, Stupid" and refers to a design principle of developing code and systems in a way that is as simple as possible to increase understandability, ease of development and maintenance. Applying KISS principles involves breaking problems down into small pieces, writing concise code to directly solve each problem, and reviewing code informally to identify issues early. Benefits of KISS include being able to solve problems faster, produce higher quality and more maintainable code, and build larger systems more easily.