What is meant by "Ground Speed" on the GPS on International Flight Entertainment systems?

I fly internationally for leisure more than most and am one of those unlucky ones that can't sleep on the flight. I quite often have the television screen when I want to try and rest set to the GPS map showing where in the world we are flying above, how many more hours I need to wish my life away for etc until the plane lands and on that one of the terms it uses is ground speed. It sometimes says ground speed 300miles p/h (or another number of miles or kilometres per hour) and yet we are flying in the air and not on the ground. Can anyone explain exactly what this "ground speed" refers to on a flight? Is it the speed the car would need to travel at in order to keep up with where the plane is flying directly above despite mountain roads etc making travel time a lot slower on the ground? Is it how fast we are travelling between one point on the map and a directly straight line to the other point despite mountains etc being below? What exactly is it?