The lift from the wings pushes the glider up and forward. This lift keeps the glider in the air; unfortunately, as the glider moves though the air, drag slows the glider down. To maintain a constant speed, the glider must therefore fly slightly ‘downhill’. Hence the glider slowly loses height over time. A typical training glider will have a glide ratio of 30:1 (it will lose one unit of height for every thirty it moves forward) while a high performance glider will have a glide ratio of 60:1 or more.

[https://www.youtube.com/watch?v=vJouxKs7T9c This video] on the CUGC YouTube channel explains this.