Double Jump is an extremely useful ability which basically describes itself. You can double jump with it. This can be very useful in situations where there are places unreachable with a regular jump.
Perk Tips Edit
If you're playing around 4 to 5 intensity, it's recommended to use Double Jump. Since the mode is very hard due to how fast the terrain gets destroyed. You can also combine it with the Fireproof perk (if you have "Second Perk Slot" unlocked) to minimize risk from lava.
Combining Double Jump with the Air Dash gives it a slight upward angle.