Unless you mean the ticks on the left hand side of the thread title. They mean there are new messages on the thread since the last time you logged in. Click on the tick and it takes you to the first unread message.
Yes the tick on the right hand side means you are subscribed to that thread (you have posted in it before) and the blue v to the left means there are unread posts and if you click on that it will take you to the first unread post. If you click on UserCP in the top right it will show you a list of all subscribed threads with new posts. I hope that makes sense!