When you are creating a new thread, you will see an input field box where you can enter separate words aka keywords aka tags. Users on the forum can click these tags, and find other threads on the forum that have the same tags.
A tag cloud is a display box that will show a cloud, or an array of various tags. These tags are pulled by popularity, meaning most used, and it looks at a timeframe, usually the last 14 days. Then, a user can see the most popular topics in the last 14 days on the forum.
If you click the Search link in the Navigation Bar up above and choose Tag Search from the drop down menu, it will send you to this link which will show you our current tag cloud:
http://wristtwisters.com/tags/
From there, I chose the tag word "racing" and this provides all threads on the forum that have the tag "racing" on them:
http://wristtwisters.com/tags/racing/
Here is the tag "tires":
http://wristtwisters.com/tags/tires/