Welcome to the Future!
The term "PredictionX" is synonomous with the full "Prediction Project," whch this web site describes. Technically, the term "PredictionX" started out as the name of an online project hosted as a set of modules (often called a "course") on the edX platform. The Predictionx.org site (where you are now!) offers: links to already-released content from the online "PredictionX" edX project; materials from the series of FS27J Freshman Seminars at Harvard; information about the upcoming on-campus course on Prediction (GenEd "1112"); as well as links to content still under development.
You might want to start with The Framework for Predictive Systems in order to better understand the Prediction project's pedagogy. From there, you will find resources related to specific parts of PredictionX (such as The Diviner's Guide and The Tools of the Navigator in Lost Without Longitude).
To get a good idea of what the full PredictionX effort at Harvard is about, and how it started, try watching one of Professor Goodman's talks on the project (see "Talks+" tab above.)