What is this Federation thing?

What do people mean by federation? What does it mean to have a decentralized system? The big barrier for new users coming to Mastodon and the Fediverse is a concept that's really quite old, but has an unfamiliar fancy name. Let's demystify it and get you started. Fediverse image is CC licensed from axbom and can be found here - https://axbom.com/fediverse/