I work in HR but have never come across this term before and Google hasn't come up with anything either so must be an informal or slang term I guess.
A supervisor at DPs work said "Oh well Dave will never leave because he has a buffer with the boss man".
It's not a very large company with no formal HR department so I would be surprised if it was some sort of formal staggered wage increase agreement...
What do you think it could be?