I know it's been a long while, but I'll get straight to the point with this, since it's a basic topic to understand. "Broken on how it is played" simply means this: "How many decks can abuse this one card?" For instance, you can have a card that can fit into some decks, but not all of them. It's also the instance where people tend to think "Broken on how it is played" equals "Broken by design". It isn't. Namely, because "Broken by design" means "What text error does this card have that makes it so broken?"
I know this lesson is brief, but I needed to get straight to the point on this issue, before moving on to the next lesson.
Also, if I have this perception wrong, go ahead and leave a comment, so that I can fix it.