“Refactor mercilessly to keep the design simple as you go and to avoid needless clutter and complexity. Make your code clean and concise so it is easier to understand, modify, and extend. Make sure everything is expressed once and only once”. – http://www.extremeprogramming.org/rules/refactor.html
To me, this principle is about writing elegant code, elegant being clean, clear, and concise, which takes an intelligent design to create.