Simplicity is the most intensely intellectual of the XP values. To make a system simple enough to gracefully solve only today's problem is hard work. Yesterday's simple solution may be fine today, or it may look simplistic or complex. When you need to change to regain simplicity, you must find a way from where you are to where you want to be.
I ask people to think about the question, "What is the simplest thing that could possibly work?" Critics seem to miss the second half of the question. "Well, we have serious security and reliability constraints so we couldn't possibly make our system simple." I'm not asking you to think about what is too simple to work, just to bias your thinking toward eliminating wasted complexity. If security concerns dictate that you split your system across two processors where otherwise you could have used one, as far as I am concerned, that result is simple. The only better solution is if you could find a way to address the security concerns on a single processor.
Simplicity only makes sense in context. If I'm writing a parser with a team that understands parser generators, then using a parser generator is simple. If the team doesn't know anything about parsing and the language is simple, a recursive descent parser is simpler.
The values are intended to balance and support each other. Improving communication helps achieve simplicity by eliminating unneeded or deferrable requirements from today's concerns. Achieving simplicity gives you that much less to communicate about.