Keyword would be easier to understand.
the "tag" word is just the same thing - whatever word or words that someone would use to do a search on.

I guess they use "Tag" because it is shorter than keyword....

Like if you had a post about a specific thing or condition you could use the Tag feature and add some specific words {main subject, symptoms, or condition} and it might help other searchers or the search bots pick up your thread and they might in turn sign up to answer you.