It's kind of a subtle distinction, but in practice you'll know it when you see it. Basically, this rule means that you can't give multiple contexts wrapped in a single clue. So "texting while driving" is a clue that might technically be "two things," linguistically, but in practice, it conveys a single idea on something like the "Dangerous - Safe" spectrum. But if you gave the clue "texting while driving a Honda Accord," all of a sudden you would need to debate the safety of BOTH texting while driving AND a Honda Accord.
This almost always just makes the game less fun and too easy, since this leads to players weighing the merits of two unrelated things against each other, then splitting the difference, rather than reaching a definite conclusion.