What does it mean that leaf nodes are where action happens?

The most concrete level of any hierarchy is where actual implementation occurs.

Example: Your goal hierarchy reads: 'Get healthier' > 'Exercise regularly' > 'Build a running habit' > 'Run for 20 minutes at 6:30am Tuesday at the park near my house.' Only the last item — the leaf — tells you what to actually do. The three levels above it are organizing principles. They explain why the leaf matters and where it fits, but none of them can be executed. You cannot 'get healthier' — you can only lace up shoes and start moving.

Try this: Pick one project or goal you are currently working on. Write it at the top of a page. Decompose it into 2-3 sub-components. Then decompose each sub-component until you reach items that are concrete enough to do in a single sitting without further clarification. Circle those leaf nodes. Count how many levels of hierarchy it took to reach them. Now ask: are you spending most of your time at the leaf level, or stuck somewhere in the middle?