What is the master / slave relationship on a device?

[eboy 0]

Hi, I know this is probably a dumb question but something I have always been aware of but never actually looked at is the master-slave listing within the setup of a device but what does this actually mean? I can see on the slave side the name of the actual device but there are also numbers (assuming id numbers) which I cannot relate to or seem to find?

Example on file for a dimmer module which I named lamp.

 

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[eboy 0]

Hi 

Just to add to this as this is showing master / slave for a dimmer module, why are there four slaves showing? On the visible panel I have a lamp and a remote widget showing against this module. What would the 47 and 47.2 be?