The hang glider can be crafted from leather, rope and wood and allows you to fly. It is best used from high places. In the first (bottom) island, the tallest mountain is north of the middle island. You can see it on your map. This is the highest point on the island, and if you deploy your glider from here, you can pretty much reach any part of the island by air. There is a teleport anomaly to the mountain top at the "stone henge" on the beach near the North-East transit point. Craft a glider before you teleport up there, you don't want to end up on top of the mountain without a glider, or enough material to craft one!
It will be cold at altitude, so dress warmly, or have lots of food on hand, because the cold will quickly drain your food meter.
Your best range is achieved if you keep the glider pointed slightly downwards while flying. If you pitch up to much, you will simply lose altitude without moving forward, and drop to the ground.
When landing, you can press the "disembark" button (bottom-middle of scree) when you are near the ground. This will pop you on the ground, and your glider will land ahead of you. If you hit the ground before disembarking, or hit a tree, your glider will be destroyed and yield a few leather resources. You may also be injured.
If you punch the hang glider with a free hand, it will be placed in your inventory.