Marion,

If you've chosen G triad (chord) as your home base, you've chosen G as your tonic. It would be a real drag to hang around there forever. (Who'd want to keep listening?) So, the most common strayings off are

1st, to the C triad, undermining (but not destroying) the tonic weight of G by using it as the 5th of of its subdominant (i.e., the pitch that lies a 5th under it, therefore "sub" dominant)

2nd, to the D triad, G's dominant (which by repeating G's 2nd overtone, seems simply to echo the G triad and give more weight to it. Because the D triad also contains the pitch F#, it's a cinch to create a demand to return to tonic G. F# is G's "leading tone" - it leads to G - & voila, there you are back home feeling very satisfied after a little trip.

And all the time you stayed in G tonic.

First vacation finished. You can study where you want to stray next time.

Hedy