StackExchange is a network of very popular Q&A sites, among others StackOverflow. Via Area 51 users can suggest new Q&A sites. A number of users must be dedicated to the site so the StackExchange team creates the site. I have made a proposal for a XenForo site. Please vote for it here.
If you still don't really know what StackExchange/StackOverflow is, here is the Wikipedia page.
Currently we need:
- 60 more followers
- 40 more questions with a score of 10 or more