I think this statement is a tautology. To understand something is to know it in simple terms. To understand something is to have mentally broken down a complex subject into its simpler pieces. For example, to understand a car engine is to take it apart in your head and know each part, how it moves, and what it does.

Many think they understand something, when really they only know how to use it. For example, I understand how to use a computer, but that doesn't mean I understand how a processor works at the level of registers and assembly language. So if I were to try to teach someone a computer, then I could say things like "Click that, and this will happen," or "Type such and such, and then this other thing you want will happen." But if anyone asked me about how that actually works, to follow all the way how a physical mouse-click gets transformed into a change in the window on the screen, then I couldn't. Or, even if I could, it might take me half an hour to explain it, depending on how much they want to know.

So maybe it's that we undestand things, but at different levels. Few people understand something at its deepest level. In fact, physicists would say no one does.