If you track a post you will get notifications whenever anyone comments on the post. If you want to stop getting notifications on a post, click the star to stop tracking it.
If you create or comment in a post you will automagically track it.
Tracking specifies you and everyone else's interest in the post over time. You might be interested in a post in the beginning and over time might want to exit the conversation. You also might just want to...

Woot!
Fantastic news.
Very cool new feature!
Thanks everyone, its been a long process of thinking about it after all that it was really only maybe a few hours to add it in.
Yay, now I won't get 1 million notifications when I comment on something. :P
All jokes aside, this is a really cool, and simple to use, idea.
Glad you guys like it! Personally I love getting notifications and only recently didn't know people didn't like that. :D
Yeah there has yet to be a conversation I wanted out of, but I can understand with things like the Terraria server, if you weren't in on it and didn't need to get updates, etc.
Looks awesome!