Items can be locked behind skill levels and achievements. By default a bunch of vanilla locks are applied, but more can be easily added through the config.
Here you can see your skill status. The skill icons get fancier as you level up, and capping a skill shows a golden border around it.
The Skill Menu, where you can level up and unlock traits with skill points. (using an edited config to demo the requirements)
An example (with an edited config) of what it looks like when an item is locked behind levels and/or achievements you don't have yet.