Car speaker frequency response means the range of frequencies that a certain type of speaker delivers best in a specific setting in a vehicle. For example, a high-end tweeter delivers very high-range frequency response so that sounds like high-range sounds are delivered clearly. A mid-range response is the range of frequencies that is heard best in a natural setting, while a low-end response is usually an amplified low-range speaker that delivers clear response at the lower end of the hearing range. High-range response is between about 6,000 and 9.000 Hz; mid-range response is from about 1.000 to 3,000 Hz.; and low-end response can vary from 100 Hz to 1,000 Hz.
For more info, see 2 Way vs 3 Way Car Speakers