My Nissin Di866 II flashgun supports operation in both "slave flash" and "master flash" mode. The former I know needs to be used with a Nikon DSLR's "Flash Commmander" mode.
What's the latter mode? Where is the difference?
Slave flashes are off-camera flashes which are triggered by a master flash (or a commander like the SU-800 or the built-in commander in many Nikon bodies).
So the master flash mode allows the Nissin to control the off camera flashes that are in slave mode.
This can be done in two ways:
The master flash simply fires during exposure, and the off-camera flashes have what is known as an optical slave mode in which they react to the flash and fire at the same time at a preset power
The master sends "pre-flashes" which control the slaves. The master may be able to control the power of the remote slaves, and may be able to separately contol groups of slaves (i.e. turn off a group from firing, or adjust the power of a group).
This nomenclature is for using multiple flashes simultaneously. In such setup, one flash is the master and all other ones are slaves: