Airspeed is the speed of the glider through the air. This is constant, unless the angle of attack changes either by the pilot applying brakes, or from a change in relative wind (see Flight Dynamics). Most gliders fly between 20 and 23 mph through the air with no brake input (trim).
Ground speed is the speed of the glider relative to the ground. Your ground speed is a combination of airspeed and wind speed. With a headwind, your ground speed is airspeed minus wind speed. With a tailwind, it is airspeed plus wind speed. The more headwind you have, the slower your landing speed (ground speed) will be. A decrease in your ground speed will proportionally decrease your glide ratio (horizontal distance divided by vertical distance), and an increase in your ground speed will increase your glide ratio. If, for example, your airspeed is 20 Mph and you have a 5 Mph headwind, your ground speed and your glide ratio will be reduced by 25% (ground speed becomes 15 Mph and a glide of 7:1 becomes 5:1).