Sound occurs when air is set into motion rapidly. Humans can hear sound if those cycles occur anywhere from twenty times each second to about twenty thousand times each second.
When a piano string is set into motion by you pressing a key, it vibrates up and down repeatedly. If the note A above middle C is properly tuned, that string will vibrate 440 times in one second. That’s what A-440 means.
Every note on a piano is tuned using A-440 as the starting point. A-440 has been accepted as the universal standard for most of the century.
Before that, there was no standard at all. If you go way back in time, every village used a prominent local instrument, such as a church organ, as the standard for tuning its musical instruments.