A glider has a glide ratio of 20/1, this can be expressed asa glide of 20 feet forward for every 1 foot down. The glide ratio can all so be expressed as L/D or 20 pounds of lift and 1 pound of drag. This ratio is fixed in the design of the aircraft but can change with the configuration of the aircraft (flaps, slats, etc).
The best L/D is found at the bottom of the drag curve and corrasponds to a given speed at a given weight. If you fly faster or slower at the same weight, your L/D and glide performance decreases.
How does weight affect the L/D ratio? Take two identical gliders with an L/D ratio of 10/1, in identical weather (no lift or sink, no winds) and forget about pilot techniques, the only differance is the gliders weight.
Glider 1 is at a max gross weight of 2000 lbs and its best glide speed (best L/D) is 70 knots. At this speed lift being created is 2000 lbs and the drag produced is 200 lbs (a 10/1 ratio).
Glider 2 is at a weight of 1500 lbs. If this glider flys at 70 knots it is producing 200 lbs of drag and it's L/D ratio is less than 10/1. How does glider 2 get back to the 10/1 ratio? Fly slower. Glider 2 needs to be at a speed to give it 1500 lbs of lift and 150 lbs of drag (a 10/1 ratio).
If both gliders are racing each other, both gliders would cover the same distance and lose the same amount of altitude but Glider 1 will do so at a faster speed. In other words cross the finish line first.
This is an over simplified version, but it should help.