When you tag an article, product or other content, you are assigning it to a taxonomy. Taxonomy is how we classify a piece of content. For example, an article about Trump winning the presidency might be tagged as "Breaking News," "Donald Trump," "Politics" and "Election." You may also want that article to require use registration or only be available to a subscriber. In that case, you'd tag it as "Require Registration" or "Require Subscription."
One benefit of tagging your content is the automatic creation of landing pages. For example, articles on the below website tagged as "Breaking News" appear on a page with news that the website can then link to for easy browsing.
To tag an article, simply select the appropriate taxonomy under Classification, Site Placement and Topic/Category when editing your content.