DBus is all about communication. The DBus daemon allows numerous processes to communicate with each other, at the same time, on the same machine. The TOP command will show numerous processes running at the same time. Without DBus, only process at a time would be able to be running.

The ps command can be used to determine if your system is using init or systemd. If PID 1 is init, then you will use the service command. If PID 1 is systemd, then you will use the systemctl command.

If your system is using systemd, use the systemctl command to determine if dbus is running.

systemctl status dbus

If your system is using init, use the service command to determine if dbus is running.

service dbus status

DBus lets parts of the system communicate with each other. When you connect a USB device (hardware) to the computer, DBus let's GNOME (software) know about the USB device. DBus let's hardware and software communicate with each other.