A standing wave is formed by two harmonic waves of equal amplitude, wavelength and frequency moving in opposite directions. When they collide, each segment of the string oscillates in simple harmonic motion (SHM). Nodes occur where the amplitude is zero, and antinodes occur where the amplitude is at a maximum of 2A. The fundamental frequency is the lowest frequency at which a string will oscillate, and higher harmonics are integer multiples of this frequency. Standing waves in instruments like flutes and guitars are responsible for producing musical notes. The document provides an example of calculating the wave speed, length, and first three harmonic frequencies of a guitar string with given properties.