Often used with HP ... if your car is lost / stolen / damaged beyond repair and the settlement figure off your standard insurance is insufficient to repay the debt (or insufficient to buy a like for like replacement after doing so by effectively giving you your cash deposit back) the GAP insurance covers the 'gap'.
Various permutations / combinations / variations / applications but in essence you get the general idea. No - never bothered with it. I never bought anything on HP, only ever in cash when I had enough to pay for it or via an unsecured bank loan without front-loading of interest, so figured unless I have a new for old insurance policy I'd just suffer the loss and put it down to tough luck.