What does it mean to follow a topic? It means that when there are updates, they will show up in your dashboard and, if you've got the notification set, you will receive an email.
There are a few ways to follow a topic. You can do so by replying to or commenting on a topic (and you're also "following" any topic you start) or by chiming in that you feel the same way or just by simply following. Allow me to illustrate:
For Questions
Click the "I have this question, too!" button and not only follow the topic but add to the urgency of the question.
For Ideas
Click the "I like this idea!" button and not only follow the topic but add to the validity of the idea.
For problems
Click the "I have this problem, too!" button and, again, you will be following it AND adding to the sense of urgency about the the problem.
For discussions
Click "Follow this discussion" and you'll do just that.
ALTERNATIVELY....
You can simply follow any type of topic without weighing in on it by clicking the "Follow" tab in the nifty Share & Follow widget on the side of the page. Click the "Follow this..." link and voila!
And don't forget about
RSS...