Look a bit like a drum but has rollers instead of shoes. One of those things that has to be dismantled to be understood.
As far as I recall a roller brake still has shoes, the word "roller" refers to the method of actuation. A drum brake would usually two shoes which were pushed onto the drum with one (single leading shoe) or two (twin leading shoes) cams. I recall the roller brake had three shoes with two rollers per shoe sitting in a moving housing. As the roller housing is rotated by the action of the cable the rollers were moved outwards by ramps in the outer casing thus pushing the shoes onto the drum. I'm told that the advantage was that it was much easier to adjust then a cam actuated drum brake.
For some reason Shimano decided to build some sort of torque limitting thingy into front roller brakes. The official reason was that it was a safety device to modulate the brakes. Some people suspect that the real reason was that if the braking torque wasn't limitted the brake would, well, break.
Although I think Simano mainly used the name to distinguish it from drum brakes that were integral with the hub, since the assembly was fitted to the hub on a splined mounting and could therefore be removed from the hub. The idea presumably being that it could be replaced without having to dismantle the whole wheel. They weren't really supposed to be servicable. When they wore out you were supposed to remove the whole assembly from the hub and fit a new one. An odd concept. After all, you wouldn't replace the whole caliper when the pads wore out.