With glide, you can make a jump mid-air without a double jump. This can be useful when you lack vertical movement options. Glide jumping does not use your double jump so you can view it as an additional jump. To perform it, while grounded, stand still and hold glide, then jump. At any point after you pass the apex of your jump, release glide and press jump again.
It is also possible to do it while running but the timing is a bit different. When running, start by jumping then hold glide shortly after. As when grounded, release glide and jump again at any point after you reach the apex of your jump to get the glide jump.