The word "wiki" comes from Hawaiian and means "fast". And that's also the principle of wiki software. A wiki is a web-based software which allows every visitor of a wiki-website to become an editor by using the easy-to-use wiki features for editing the contents of the website. In other words: A wiki gets its energy and quality from the participation of its users who are not only able to find informations but also to create and update them. From different places and at different times they create the contents of a wiki collaboratively.
The best known wiki is the online encyclopedia Wikipedia. Wikipedia is based on the software MediaWiki - the same software which is used by GrassrootsWiki, too. That means: If you know how to edit articles in Wikipedia you're already skilled to create and change articles on GrassrootsWiki, too. If not, you learn it fast by using the 
edit buttons and, if you want, the sandbox for practicing.
Learn more about Wikis on Wikipedia.