In a powered aircraft, the thrust from the engine opposes drag. The glider, however, has no engine to generate thrust. With the drag unopposed, the glider quickly slows down until it can no longer generate enough lift to oppose the weight. The simple answer is that a glider trades altitude for velocity. It trades the potential energy difference from a higher altitude to a lower altitude to produce kinetic energy, which means velocity.
Gliders descend relative to the air in which they are flying.
The answer is that they are designed to be very efficient, to descend very slowly. If the pilot can locate a pocket of air that is rising faster than the glider is descending, the glider can actually gain altitude, increasing its potential energy. Pockets of rising air are called updrafts . Updrafts are found when a wind blowing at a hill or mountain has to rise to climb over it. Updrafts can also be found over dark land masses that absorb more heat from the sun than light land masses.