Is a message bus system, a simple way for applications to talk to one another. In addition to interprocess communication, D-Bus helps coordinate process lifecycle; it makes it simple and reliable to code a "single instance" application or daemon, and to launch applications and daemons on demand when their services are needed.
https://www.freedesktop.org/wiki/Software/dbus/
https://www.freedesktop.org/wiki/Software/dbus/