This is probably the most asked question at any glider field! The answer is that a sailplane flies as happy in a sky with no wind at all, the only "wind" we need is the air rushing over the wings to generate the lift to sustain the sailplane and the "speed" of this air is controlled and maintained by the pilot, not by the wind in the sky.
After launch, the sailplane sails through the air, and descends to generate lift. The challenge of the sport is to remain aloft. To maintain or increase altitude, the sailplane must be flown into air which is rising at a rate equal to or greater than the rate at which the sailplane is descending.
This is the most common type of lift. A thermal is an invisible mass of rising warm air, formed by the heating of the ground by the sun. As the sun heats the ground, the ground in turn heats the air above it. As the air gets warmer, it get lighter and wants to rise. When the temperature difference between the air and its surroundings is great enough, the pocket of air will break away from the ground and rise (similar to a hot air balloon rising). The strength of the thermal will vary with the intensity of the sunlight, the composition of the ground and the temperature differential. The maximum altitude varies dependent upon the level at which the warm air condenses (cools). If the air is moist enough, the end result of the condensing air is a puffy, white, cumulus cloud.