@LensaMoon@mastodon.art A service handler is a rule that tells your web browser what to do with links that start with a prefix other than “http:”or “https:”. Gmail can install a service handler so “mailto:” links open in Gmail, for example.
Presumably, Mastodon handles some kind of protocol now? @Gargron would know. Maybe it’s “toot://“, but probably not. It means that various links might ask Mastodon to do something, if you choose to install the handler.