I have to admit I had no idea either, but if you go to the first image in the register book you're looking at and go forwards a few pages, you'll find a key which explains the various abbreviations, which I guess will be the same as the ones I found. Basically these show how the person is qualified to vote, so for men:
R = Residence qualification
B = Business premises qualification
O = Occupation qualification
D = Qualification through spouse's occupation
Rw, Bw, Ow and Dw have the same meanings but apply to women.
NM = Naval or military voter
I hope that helps!