Skills, essence and perks
Every skill levels by doing the thing — farming by harvesting, mining by breaking rocks, and so on. Levels alone unlock perk tiers; buying the perks themselves costs essence, the purple motes that drop from almost everything you interact with.
Essence is shared across all skills, so spending it on one tree is spending it against every other. That is why the costs are worth reading before the first purchase rather than after.
Every tree opens on the same ladder — tiers become available at skill level 1, 15, 30, 45 and 60, shown against each tier below. Reaching the level opens the whole tier at once; it does not buy anything. Each perk in an open tier is still bought separately with essence, so a freshly opened tier 3 is five decisions, not one.