I had to research this question a bit before having the slightest idea what tags are or how to use them. And now I STILL have questions.
When you start a new thread, you can add tags to it. You might tag it as "log cabin" or "vintage machine" or whatever you like. You can also search for all threads with a given tag. However, you can also leave that field blank. I don't think most of us are using this feature, because we don't know about it. My question is whether there's any way for the OP to apply tags to a thread after the thread has been posted.