"Keep It Simple, Stupid" (or "Keep It Simple, Silly" for more polite notes) is a principle that is often used when trying to solve a problem or create a design. At the core of KISS, simplicity triumphs over complexity, and the trivial, straightforward approach is often the best one.
It should be noted that KISS is not always the best approach. Sometimes, the optimal solution is complicated and unintuitive. Algorithm development is a very good example of this: The KISS approach is usually called the "naive" approach, and more complex algorithms often solve a problem faster and more space-efficient.