A wiki is a kind of website. It's designed to allow a community of people work together to share information about their shared project. It embodies an extreme philosophy of openness and collaboration, because it allows anybody (yes, that includes you) to edit what's on any page.
At the bottom of every page you can see a link called "Edit text of this page". Click on it, and you'll see that you can edit these words.
Probably this all very confusing, so read a bit about wiki before you start to edit this one.