What is 440 Hz?
First of all, Hertz (Hertz, Hz) refers to the number of cycles per second. Usually, when tuning an instrument, the La (A) sound is used as the basic sound.
Therefore, when we say A=440 Hz, we mean that the La (A) sound wave above the center C (yellow in the figure below) vibrates 440 times per second.
A440 is the musical pitch corresponding to an audio frequency of 440 Hz, which serves as a tuning standard for the musical note of A above middle C, or A4 in scientific pitch notation. It is standardized by the International Organization for Standardization as ISO 16.