A thread is a thin string, like you use for sewing. In Esperanto, fadeno can be thread or yarn, which is thicker and used for knitting.
A forum thread is a series of public messages. It's as if they're all sitting on a string in certain order.
Edit: I noticed this morning half a dozen wisecracks and "accepted" posts. Those are really frustrating for peple who are trying to answer questions. I see you do occasionally have questions. I would like to ask you as a favor if you want people to answer your questions that you cut back on the clutter posts.