The notes of A6 are A, C#, E and F#. A is the root, C# the major third, E the fifth and F# the sixth. Said aloud, its notes are A, C sharp, E and F sharp.
From the low string to the high one you hear A, E, A, C# and F#. A rings twice across the five strings, and it is the root. The lowest note is A on string 5 and the highest is F# on string 1.
In A6, string 5 is played open and adds A. String 6 is not played. Every finger presses at fret 2. The second shape on this page starts at fret 5.
A is one change away. String 1 is left open, bringing in E, the fifth of A. A6 holds the whole F#m chord inside it, A, C# and F#. Going to Bm, one position stays the same.
A6 belongs to six keys. In A major it is the I6, in D major the V6 and in E major the IV6. The rest appear in the key list.