Bass, mid, and treble are the three primary frequency ranges in audio/music/audiobooks. The bass (low frequencies) are 20Hz - 250Hz, the mid range is 400Hz - 5.2kHz, which are generally associated with musical instruments like guitars and tom drums and the treble is 5.2kHz – 20,000Hz, which is typically associated with high-frequency sounds such as cymbal crashes.
An Equalizer, or EQ is a key feature that allows you to adjust or tune the volume of these frequencies in an audio system. It's a feature that is often found on speaker systems, amplifiers, mobile phones and even certain music applications. Adjusting the bass, mid, and treble settings can allow you to fine-tune your music/audio to your liking and to help eliminate any issues you may have with your sound.
A lot of car audio enthusiasts and hobbyists try to achieve better sound by aiming their speakers, but they don't always succeed. If your tweeter is aimed incorrectly, it can distort and eventually blow. The physics behind the direction of a tweeter's directivity has been laid out in an earlier article.
For the rest of the speakers in your audio package, it's a good idea to keep them flat, which is the way that the audio engineers that work on new car stereos set them up. If a sound engineer sets your system with a treble-heavy equalization, you'll hear the music's high-frequency elements, such as the cymbal crashes, but you won't be able to hear the bass and midrange parts of the song.