Tags are a cool new feature in IP.board (well, new to us! I don't know when they implemented them). Tags are a fairly common "web 2.0" feature, allowing you to categorize things multiple different ways simultaneously.
You can add tags, up to 10, in the area under the "Topic Title" section when posting a new topic. They can be separated by commas (you can use spaces in tags, so you need to separate them with commas). If you've already made a thread and want to add/edit tags, you can do so in the "edit post" screen, by using the Full Editor.
Selecting the box "Use first tag as prefix" will highlight the first tag, and will place that in the forum list in front of your topic title.
Effectively tagging topics can enable much more powerful (and easy!) searching. Clicking a tag will start a search of all posts that use that tag.
Tags are entirely optional; if you can't think of an appropriate tag, don't worry about it!
An example of good tagging would be a thread announcing a new Firefox release. You could tag it "Firefox" "release" "news" and possibly "update".
When we build up a base of tagged threads, we can see "also tagged" threads at the bottom of posts.