What's something in a subject you had difficulty understanding, and what (alongside practice) helped you better understand it?

Routing tables. I thought I understood them until I suddenly found my self in charge of setting up a rather large network with multiple routers and VPN tunnels.

Nothing teaches you proper routing like having to juggle 50 static routes.

I'd have to give credit to amateur videos about economics for helping me understand it better than actual experts giving expert lectures on it.