You now have two G notes; the one from the open third string (previous diagram) and the one at the third fret on the first string.
This type of repetition occurs with all notes, since the musical alphabet goes from A to G, and then back to A again. The distance between the two G notes is called an octave.
G
To play the G chord, place the first, second and third fingers of your left hand as shown in the diagram. Strum all six strings.