RSS is a "really simple" technology that can help you keep track of changes in ProjectForum forums (and all kinds of other websites), without having to constantly visit the site to see if anything has actually changed.
Many web browsers today (e.g. Firefox, Safari) have built-in support for RSS, so you can use them to keep track of changes in your forums. There are also dedicated applications called "RSS news aggregators" that let you use RSS to watch for changes. These run either as applications on your own computer, or as a service offered by a few websites.
An RSS "feed" is associated with each forum (and in fact, there's also a feed associated with each individual page). To track changes, you'll need to "subscribe" to one or more of these feeds, using your RSS-capable web browser or news aggregator. This can usually be done by clicking on the orange RSS icon in the "Notify" pages; you may instead have to copy the address of the feed from the page, and paste it into your browser or aggregator.
Your web browser or news aggregator will then take care of watching for changes in all of the forums or pages whose feeds you've subscribed to.
You can find out more about ProjectForum and RSS at the ProjectForum web site.