The glide slope is the ratio of forward run per unit length descent. Most often used to describe sailplane performance, it is equal to the lift-to-drag ratio and is a good thing to know if you are forced to make a dead stick landing (unpowered due to engine failure).
Higher glide slopes mean greater efficiency, and in an unpowered aircraft, translates directly into longer times aloft. It also means that the pilot, with the use of spoilers, has much more control over his/her rate of decent and thus a larger margin of error for landing.
The glide slope depends on the relative velocity of the aircraft and wind, and peaks at a certain speed called the minimum sink.