UX is like a joke. If you have to explain it, it's probably not that good*.

Unknown author

UIs should be simple right?

It’s not actually strictly true. Boeing 747s interfaces need years to explain and train, but I’d argue nobody dare say they aren’t very good. Mihaly Csikszentmihalyi came up with Flow theory and it makes that case well for complex interfaces.

But in general, affordance is an important principle in design, the small clues that usher us to do the right thing. It can be bleeding obvious or can use gestalt laws to nudge you to the right thing. 

I assume whoever wrote this quote assumed we understood whom we were explaining it to (the user) because often explaining the nuances of a UI to your stakeholders ( why your list is exactly 5 items, or why you have a breadcrumb ) is to explain the psychological principals underpinning your designs.