It is a link to a yet non-existing page and usually marked in a special way, e.g. red-coloured or with a question mark, so that people immediately recognize that this page hasn't been created yet. In this way you can encourage the growth of a wiki because people often don't know where to start in a wiki or what information might be needed. The links indicates the lack of special information clearly.
When editing a page use FutureLinks whenever it comes to your mind that you could to link another piece of information and that information has not been created yet. In the first place these links are a good reminder to write something down; it can encourage others with better knowledge about that special topic.