What is meant by adding a tag?

A tag is a "keyword" for the search engine that catagorizes discussion topics for people to browse and choose.

When the tag window is open along with discussion page, you can put the key word which can define about the subject of the topic. Like if the discussion is on Cool drinks, you can put in the tag column any of the following or all of the following words, separated by a coma, Pepsi, Coke, Orange Juice, Water This will help mylot to categorise the discussion/responses.