It’s still a note in the G chord (G-B-D), but it means you’ll often find chords and riffs phrased around an open fifth-string root. The tuning is common in many styles including Hawaiian ...

To do this, tune into open G by dropping the sixth, fifth and first strings down a tone, then drop the second string down a semitone to a Bb – that’s the all-important minor 3rd in the key of G.