Web sites that accept postings from the public, like this wiki, are often abused by spammers who use automated tools to post their links to many sites. While these spam links can be removed, they are a significant nuisance.
Sometimes, especially when adding new web links to a page, the wiki may show you an image of colored or distorted text and ask you to type the words shown. Since this is a task that's hard to automate, it will allow most real humans to make their posts while stopping most spammers and other robotic attackers.
Hit the 'back' button in your browser to return to the page editor.
We also occasionally use simple math problems, instead of a standard image reading as CAPTCHAs.