For example, I love the game ufc-undisputed-2010 and I would like to know if any new questions are asked about it (I bet I can answer most of them). Nowadays I need to manually check the tag once in a while (and it only has a few questions).
I'm not going to check that tag since it is not popular here (the games has its own forum). But it would be cool if I get some information if new questions in that tag are created.
If you hover your cursor over any tag, a box with a description of the tag appears. In that box are two links, rss and subscribe that allow you to monitor new questions. "subscribe" is for email notification, "rss" for subscribing to an RSS feed.