I decided to give Mastodon a whirl, so a while back I created an account for myself at the icosahedron.website instance. (After all, a big part of my research is to generalize regular icosahedra to higher dimensions and complex coordinates.) There I am: Blake C. Stacey (@firstname.lastname@example.org). It’s been fun so far.
It seems the best way to explain Mastodon to an old person (like me) is that it’s halfway between social networking, the way big companies do it, and email. You create an account on one server (or “instance”), and from there, you can interact with people who have accounts, even if those accounts are on other servers. Different instances can have different policies about what kinds of content they allow, depending for example on what type of community the administrators of the instance want to cater to.
If I ever administrate a Mastodon instance, I think I’ll make “content warnings” mandatory, but I’ll change the interface so that they’re called “subject lines.”