The skill tree is a way for players to progress their abilities in-game by spending skill points earned by doing missions on perks. These perks include things such as upgraded pickaxe, more storage in the Storm Shield, etc. This is broken up into 5 categories, Core, Building, Ranged Combat, Scavenging, and Melee Combat.
The Skill Tree also includes Research Tiers, which grant the ability to unlock bonuses with Research Points (gained over time), to affect F.O.R.T. attributes (Fortitude, Offense, Resistance, and Tech). These include Backpack upgrades, and additional power upgrades to Vehicles for Expeditions.