im new...what is a tag?

Welcome! I am not sure ~ where did you see this?

Tags are keywords. A look at the tags should tell you what a thread is about.

They can be assigned to threads and if used consistently allow for quicker and more accurate searches, because a tag search does not scan the words used in the comments, but instead only the tags, the keywords.

You know, when a topic derails and lot's of sidetalk happens, it is possible to get completely off topic search results, because the normal search cannot differentiate whether a word in the comments is important or not, tags eliminate this issue.