Eh, not being able to explain it elegantly on the spot in front of people understandable.

But if can't come up with a simple explanation even when you're by yourself with all the time in the world, then yeah you probably don't understand it well enough.

It's for people that say "it's a complex topic. You wouldn't be able to understand it". Bitch, the human brain is amazing. People can understand anything they want to. You're just too lazy to help them.