It means that if I pick up a random USB cable and plug it into a USB port I have no idea how well it will work or even if it will work at all. It's like the U in USB stands for Unpredictable.

This article we are discussing this about gives a great example of why understanding the difference matters a lot when purchasing hardware.

A sense of perfect knowledge of the things around you, and not feeling like someone is trying to take advantage of you any time you go shopping.

Well it means I have no idea what TFA is talking about when it mentions USB versions, for one.