RSS stands for Really Simple Syndication. It is a way for you to get the latest content from this community by using an RSS feed reader of your choice.
After you have your feed reader set up, you can find RSS feeds in the community by going to a board, blog, thread, or post and selecting Subscribe to RSS Feed from the Options menu.
There you will see a preview of the feed. When you connect the RSS feed to your reader, the RSS feed will appear in your reader as well as new content from that section of the community whenever it becomes available.