A thread is a sequence of related messages on a bulletin board (like this one). An impact on SEO arises when a message contains a link (known as a "backlink"). The impact can be positive or negative depending on a number of factors. E.g.
- The ranking of the site linked from.
- Whether the link is "do follow" or "no follow".
- The relevance of the site linked from.
- The relevance of the thread.
If you search Google or Bing for "SEO BACKLINK" or some such, you should find plenty of advice on the use of backlinks.