It is really beyond the scope of this forum to explain how RSS works. But I'll try. RSS stands for Really Simple Syndication. Think of it as a newspaper that you subscribe to and eveytime there is new news, you get that update with a date and time stamp. A RSS feed is that update. There are two modes of operation. One is intenal to hub pages and the other is external.
The internal feed is where you can get a feed from someone elses hub page that will give you a feed their hub page has been updated. The external one will give you a feed when a website that you have subsribed to sends a feed to your hub page. For example, If you subscribe to the FDA RSS feed on nutrition, every time there is an update to that FDA page, you will get an update on your hub page that has that RSS link. Therefore you can keep your hub pages up to date and promote it with a RSS feed. To describe how to implement it requires a fairly lenghty procedure. I have a hub page that tells you how to do it, but I can't give you the link to my hub page. It's against the rules. But you can go to my hub pages and find it. I hope this gives you some insight into RSS.