I'm not makbike, but a pannier is one of these.
They are basically like the saddlebags on a horse fitting over your rear rack or the front wheel.

I have a handlbar bag that I put on my road bike, a Trek "Bike 'n Hike" one that can convert to a fanny pack. It fits well enough, but I haven't used it for commuting and I don't know if it would be big enough for that purpose.