Fosstodon is a Mastodon instance that is dedicated to Free & Open Source Software, or FOSS as many call it. Fosstodon is for people who are interested in open source technology and the awesome community around it. Whether that be Linux, open source software like Firefox, or Mastodon itself. If you're interested in technology, Fosstodon is a great place to start.
Mastodon is a social network based on open web protocols and free, open-source software. It is decentralized like e-mail.