So far, we have no idea what powers the Mayor might be given, nor how he (or just possibly she) might be got rid of, nor much else. That's all to be decided later, apparently! But whatever the dispensation, it's now been pre-approved by the electors of Bristol - or rather, by just under 13% of them, which was enough.
So far, only Bristol has voted in favour. I'm not sure whether it was also uncoincidentally the only city where a referendum was being held but not council elections, but I wouldn't be terribly surprised.