A G Major 6 (G6) chord contains the notes G, B, D, E. It is built from the major 6th formula 1 - 3 - 5 - 6, with G as the root.
How do I switch between G and G6?
In the common open G shape your ring or little finger frets the high E string at the 3rd fret. Lift it and the string rings open at E, turning the chord into G6. Because nothing else moves you can alternate between the two mid-strum, which is a familiar sound in folk and country rhythm playing.