Pure gliders are unpowered aircraft so they need to fly in rising air to stay aloft. Imagine a paper dart flying inside a lift. When the lift goes up so does the air inside it and so does the paper dart! The paper dart is still descending according to someone in the lift but it's climbing according to a ground observer. Maybe this is why we call rising air "lift”
Note that gliders are proper aeroplanes with conventional controls (essentially the same as a Spitfire’s but with no throttle and fewer guns) Proper gliders should not be confused with floppy things that require the pilot to use his own legs as the landing gear, like hang gliders or paragliders.
What are the main sources of lift?
Thermals are bubbles of warm air that float up from sun-heated patches of ground and rise until they cool down enough to disperse or sink again, at bit like the blobs in a lava lamp. By circling in these rising bubbles of air gliders can climb along with them.
Hill or Ridge lift is created by the wind blowing up the face of a hill or a ridge. Gliders can stay aloft by tacking back and forth along the area of lift in front of the hill or the ridge. For the ultimate thrill just use a mountain!
Wave lift can form, in the right conditions, when the wind curls over the top of a ridge, bounces off the ground, then climbs again in a series of ascending waves which can reach many miles downwind and thousands of feet high. Gliders can climb in each wave front and hop between them to continue the flight.
Using one or more of these methods, gliders can fly hundreds of miles by climbing in an area of lift and then gliding to another one that lies more or less in the required direction.
Finding that next precious lift source is one of the key challenges in gliding.