Idea of the Breath

I heard a few times the notion that in meditation one must focus on the sensations of breathing (which seems to make sense to me) but not the *idea* of the breath. What would be the 'idea' of the breath? Is there a single thought or mental entity one could focus on? I feel like thoughts change and follow one another quickly, I don't understand how one could try to meditate on the 'idea' of the breath. **What does such an idea refer to?**