A feed is a format for delivering frequently updated content to readers. RSS, otherwise known as Really Simple Syndication, is one way to deliver feeds to readers. There are feeds for all kinds of things. You can get feeds for weather updates, for "This Day in History", for comic strips, and even for google searches. They are getting more powerful all the time. But the most common use of RSS is to share blog feeds.
So how do you "subscribe" to a feed or read a feed? Well of course you need a "feed" reader! :)
There are tons of feed readers out there, but they basically break down into 2 camps: Desktop Readers and Web-Based Readers.
Desktop Readers download content onto your computer. This way you can take what you're reading with you wherever you go. You also can search these feeds quickly. The downside is, if you are low on space, over time desktop readers may really start to eat some space. Some common desktop readers would be SharpReader (the first feed reader that I ever used), FeedDemon, RSSOwl, and even built-in browser feeders for Internet Explorer 7 and Firefox and Safari.
Web-Based Readers on the other hand may be somewhat slow, depending upon your connection. The upside is they won't take up as much space, and they will probably "interface" well with other web products. Bloglines has some really nice features, but Google Reader is my favorite because it works well with Gmail, Blogger and even Google Talk. And Google Reader has tons of quick keyboard shortcuts.
Once you know what feed reader you want to use, then you have to add a subscription to a feed. On most pages that serve "feeds" you'll see an icon similar to the one at the top right of this post. If you click this, it will open up the address for the feed subscription. Just copy and paste that address into the feed reader of your choice and you should be good to go.