What does the "6" mean in "Bb6"?
It means to add a major sixth above the root of a major chord. Thus
Bb6 is a
Bb major chord plus
Bb-D-F-G, as others have already explained.
The "Bb6 arpeggio" instruction
This is an instruction to the pianist that while the voice part is holding its
G (m. 7, beat 5), the piano should play an improvised
Bb6 arpeggio as a flourish to decorate and highlight the voice.
The notation is not a common one, but the device of placing an arpeggio like this at a musically significant moment is very common.
The initial harmony, on beat 4, is
Bb7 — the "7" being the
Ab in the voice and piano right hand. However, the arpeggio is to occur when the voice moves to
G, so a
Bb7 arpeggio would create a clash:
G. Rather, the arpeggio is
Bb6 to reinforce the voice's