The "E string" refers to the highest string on a standard guitar, which is tuned to the note E in the second octave. It is the thinnest string, making it easier to play higher notes and melodies. The E string is crucial for various playing techniques, including strumming, plucking, and soloing. In addition to guitars, the term "E string" also applies to other stringed instruments, such as the violin and cello, where it represents the highest pitch string. Understanding the E string is essential for musicians as it forms the foundation for playing scales and chords.