If you are not logged in you will just get "today's posts" which is the list over the last 24hrs of new posts.
If you are logged in (look for user name at the top) you will get something smarter "new posts" which only lists threads with new posts that you have not seen.
If you click on a thread and read it it will disappear from new posts, (but it remains in today's posts) until someone posts again with something you have not seen. A great feature and normally the best way to view the forum.
Think of "new posts" as a smart version of "today's posts" that knows what you have already read.
If you want to revisit threads that you have already read (and have therefore disappeared from "new posts" click on "portal" (top left hand button) and they will visible again.