The tags are a way to find posts by topic. The tags are entered by posters in the "edit tags" line. For instance, one of the threads in the football forum about the Chargers-Broncos game was tagged "fan boys," "get over it," and "howler monkeys."
If you see these tags on a post you can click on that tag to see all posts with that tag. For instance, say we get some more whining after other controversial plays in the NFL. A poster can tag those threads "fan boys" as well, and when you need a laugh you can click the "fan boys" tag at the bottom of the thread to see more threads like it. The search function is now set up to search for tags as well.
I have seen tag systems like this used in many blogs so readers can find entries by topic. Those tags can entered by the author of the blog or the readers themselves.
I do not know what "uml" means or why a poster would enter that as a tag.