To build the motor to withstand a lot of power.
Basically overbuilding the motor for it's use is makingit bullet proof, it's just a term used to describe it's strengh.
And example would be if you were building a box out of wood to ship a computer. Using 1/4" plywood and 1x4's would be more then enough, but if you wanted to build a "bullet proof" box you would build it out of 1" plywood and 2x4's to make sure it won't break