This part of the Handbook is Work-In-Progress.
mbus is the managarm message bus system. mbus is used by servers to publish objects, which other servers can discover. If a server finds an object of interest, it can then use mbus to connect to that server. Further communication happens over the more common hel streams documented in the hel folder of this handbook.
The code for mbus can be found at https://github.com/managarm/managarm/tree/master/mbus.