B♭6 means B♭-D-F-G (the formula for a major 6th chord is R, W+W, W+H, W, where R is root, W - whole tone and H - semitone).
As for arpeggios and how they are notated - I do not know for sure. I play 2 monophonic instruments and rarely deal with this sort of symbols. I think notation depends on time period and what instrument it was written for (I vaguely remember arpeggios for guitar being notated in a different way).
It's pretty common for jazz and some rock scores to prompt the musician to play a chord or its variation instead of what's on the sheet (it's the first time I see something like that in a classical score). I would assume it prompts you to play an arpeggiated major 6th with your left hand (the bass) instead of 2 B♭'s an octave apart. This is how I would interpret it: