Tags are words and phrases used to describe a topic. These are used for filtering topics in the forums to make them easier to find. For example, if you have a sports forum a topic can be tagged with things relating to Basketball or Football, this way users have an easier locating their Topics of choice.

Admins and Post Authors are capable of adding tags to the Lead Topic.

Tags can be found forum level in a "phrase cloud" above the forum.

To learn more about Tags visit Wikipedia:
http://en.wikipedia.org/wiki/Tags