What's a "tag" when submitting a thread please

I'm trying to post up a thread on a car forum and it keeps telling me that I requested a tag which is too short (apparently I'm allowed 5 tags, as if I wanted them ) where the blooming heck would a tag be put even if I wanted one then? I just want to post up my bladdy thread on there. I've put a proper title and I've put a message, it's exactly the same as it is here on Dogsey, but what's with this tag business anybody know please???? Thanks! Grrrrrrr

Did you ever find out what a tag is?, as I would be interested to know too.

a tag is a search engine phrase

I did find out Bev and thanks Krusewalker. There was half a dozen set "tags" in a box underneath where you post, and I had to delete all of the ones that didn't have five letters in!