The idea: tools sorted by concept depth — how much you need to understand to really use each one — from Level 0 (use ready-made AI, no theory) to Level 4 (research- and production-scale systems). It's not about how hard a tool is to click; ChatGPT is easy to use (Level 0) even though the ideas behind it are advanced.
Every tool has two links: 📖 Read — an approachable article/tutorial to understand it — and 🐙 Code — its repo or homepage. Levels are judgment calls; you might move a tool ±1 level, and that's fine.