Partly updated for Enshrouded 1.0— 1.0: the unlock cost; the rest is pre-1.0 wiki data

Double Jump
Allows jumping a second time while airborne.
Where Double Jump sits on the 1.0 tree
In the Core area. It touches a starting point, so it costs 2 points to reach.
Related Ranger skills
Same archetype — likely candidates to slot next to Double Jump in a build.
Airborne1pValues in the 1.0 files: 12% per rank, 3 ranks.
Arachnoid1pValues in the 1.0 files: 20% per rank, 3 ranks.
Backstab Mastery1pValues in the 1.0 files: 20% per rank, 3 ranks.
Blessed Arrows1pValues in the 1.0 files: 8 per rank, 3 ranks.
Counter Battery1pValues in the 1.0 files: 8% per rank, 3 ranks.
Dexterity1pIncreases your Dexterity attribute by 1 Increases Bow and Dagger damage by 5% per Attribute Point
Eagles Bane1pDamage against flying enemies increased by 30%.
Endurance1pIncreases your Endurance attribute by 1 Increases Stamina by 10 per Attribute Point
Sources: game files 1.0 + Keen Games 1.0 patch notes (changelog, combat changes) for the unlock cost; enshrouded.wiki.gg (pre-1.0) for the description and effects.