A A Major 6 (A6) chord contains the notes A, F#, C#. It is built from the major 6th formula 1 - 3 - 6, with A as the root.
How is A6 different from A major?
One note. In open A the D string is fretted at the 2nd fret, sounding E — the 5th. In A6 that finger moves up to the 4th fret, turning the E into F♯, the 6th. Everything else stays where it is, so the two chords sit a single finger movement apart and can be alternated within a bar.