from man hald
The HAL daemon detects devices present in the system and provides the org.freedesktop.Hal service through D-BUS.
While this is your answer, I'm going to guess it may not make any more sense to you. I don't know that I can explain it very well, so hopefully someone else can chime in.
Basically, it is scanning the system to see what devices are present, and sends that information to Dbus. Dbus is a way for processes to communicate with each other.
Below is a snippet from wikipedia on dbus, if it still doesn't make sense, hopefully someone more knowledgeable than I in this area will chime in.
D-Bus is an inter-process communication (IPC) system with three architectural layers:
* A library, libdbus, that allows two applications to connect to each other and exchange messages.
* A message bus daemon executable, built on libdbus, that multiple applications can connect to. The daemon can route messages from one application to zero or more applications.
* Wrapper libraries based on particular application frameworks.
D-Bus is designed for two specific cases:
* Communication between desktop applications in the same desktop session; to allow integration of the desktop session as a whole, and address issues of process lifecycle.
* Communication between the desktop session and the operating system, where the operating system would typically include the kernel and any system daemons or processes.