Reference Sync Generator - Education Please?

Hi All

Pardon my "basic" confusion here but would some please explain the need for a Reference IN when setting up NLEs including the Kona or IO? Why do we need it?
If we need it, how can you setup such input. I heard/read Black Burst Generators etc.
Is there any way of setting it up without the use of any other additional equipment?
I remember them vaguely (TBC's) in the Amiga days but never thought of these when switched to Apple.
Is there any reading on the Net or sites where I can get more info about setup and detail descriptions?
Thanks
Sam M.