snac2, currently available in Debian testing and unstable, is described by its upstream as “A simple, minimalistic ActivityPub instance written in portable C.” It provides an ActivityPub server with a bare-bones web interface. It does not use JavaScript or require a database.
ActivityPub is the protocol for federated social networks that is implemented by Mastodon, Pleroma, and other similar server software. Federated social networks are most often used for “micro-blogging”, or making many small posts. You can decide to follow another user (or bot) to see their posts, even if they happen to be on a different server (as long as the server software is compatible with the ActivityPub standard).
In addition, snac2 has preliminary support for the Mastodon Client API. This allows basic support for mobile apps that support Mastodon, but you should expect that many features are not available yet.
If you are interested in running a minimalist ActivityPub server on Debian, please try out snac2, and report any bugs that you find.