Popularized by Wikipedia, wikis aim to be a comprehensive reference material to either specific or general subjects. Wikis typically consist of lots of individual articles, updated by a team of volunteer writers.
Wikis are frequently hosted using open source software due to the open source nature of HTML; a popular example amongst these projects is MediaWiki, being repsonsible for Wikipedia's backend.
GitHub allows for each repository to host a wiki of their own, allowing for detailed documentation of any repository's function. GitHub wikis may be relevant to continued development, relevant to those utilizing a repository's application, or a repository's entire purpose.