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're places too high to jump with a regular jump. You can likely see double jump players in intensity mode.
Perk Tips Edit
If you're playing around 4 to 5 intensity, I recommend you to use Double Jump. Since the mode is very hard due to how fast the terrain gets destroyed. I also highly recommend to use fireproof if you have the "Second Perk Slot" Since Double Jump and Fireproof go very well.