As you may know the hz (short for 'herz') value denotes the frequency of a pitch by the number of oscillations per second.
The 'A' in A440/A=440 hz refers to the A above middle C (shown below) in a standard (or 'concert pitch') musical score.
This note is the one most commonly used as a reference when tuning instruments which are meant to play together in an ensemble or orchestra.
The average tuning fork also sounds this pitch. So for example, if you wanted to tune your guitar using a tuning fork, fretting the 5th fret on your high E string(/1st string) should sound in unison with the note produced by the tuning fork.
The frequencies used for all other notes (on modern instruments) derives from a system called 12-tone equal temperament/12-TET, where all semitones and intervals of the same name have the exact same ratios regardless of where they appear in relation to the agreed upon basic frequency of A=440. You can find out more about the specifics here:
https://en.wikipedia.org/wiki/12_equal_temperament