1. WHAT IS resonance
2. whAT to frequncy of a sound when a car approaches
How is resonance used? One kind of resonance is chemical resonance in which several structures can be drawn to represent a compound an the "real" structure is a hybrid of all of the possible structures. Another kind of resonance occurs, for example, when a sound resonates in a cavity. Your second question doesn't make sense. I think you omitted some of the words.
I don't know how this happened but I posted the above response although it says Anonymous posted it.