It's a term that Noodler's uses to name their inks that are waterproof, tamper proof, and permanent among other things. Someone more educated on it can chime in, but it basically means that it's nearly impossible for someone to mess with it once it's on paper. Not all of their inks are "bulletproof" but the ones that are will be labeled as such.
Nathan uses bleach, solvents, alcohol, and UV (among other things) before proclaiming them Bulletproof.