SHM stands for Simple Harmonic Motion, which is described as a motion in which the restoring force is proportionate to the body's displacement from its mean position.
This restoring force always moves in the direction of the mean position.
The letter is frequently used in physics waves to signify the period of oscillation (sometimes known as simply 'period').
The time between one wave and the next passing over the same area is known as the period of oscillation. It is commonly stated in seconds as a unit.
The frequency of the wave is the inverse of the period of oscillation.