What LeeRoyMan said (New Posts) will show only new threads that you haven't read yet (it will not show threads that have new posts if you have already looked at the thread in the past).
If you use the "Unread Items" you'll see only things that you haven't read yet.
Also, you can adjust how far back they go (Spoler: it's not every thread on file, it's defaulted to the maximum of 30 days)
Example:
If you select "New Posts" the above item would not show. If you select "Unread Items", then it will
Pick what you want, because it sounds like some combination of these 3 things is what you're looking for.
Personally, I like the pulse the way it is, as it's configurable to do what you want. It's the only way I browse through the site, and that includes when I go back to a thread to post on. So I keep it set to All Activity, so I can go through and find threads that may not have new activity since I last looked, but want to post on.