The Skill Tree is unlocked once you've reached the Nether.
Once unlocked, this menu can be found by tapping the Shield button in the lower middle.
The Skill Tree is a new system added with PickCrafter version 3.0.
This allows you to spend your picks to unlock and invest points into skills that boost your capabilities.
There are 3 Skill Trees to grow! Player, Chest, and Idle.
You unlock Skill Tree points by spending your Picks on them, every subsequent point will cost more Picks each time.
You then use these points to invest in any of the three trees to improve your stats.
Once you Prestige, you will have the opportunity to transcend the Skill Tree. Transcending allows you to unlock more points for select Skills within the Skill Trees!