What is an RSS Feed?

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I have tried to find the answer, but can someone explain this to me in layman's terms. I am not familiar with this and don't understand all of the techno mumbo jumbo. Is there a good reason to have RSS feeds? What purpose do they serve? How do I get them? Do they cost anything? Are there free ones?? How do I add them to my blog? Any information will be helpful. Thanks.