don't mind nook saying things like that, hes trying to be smrt
RNG = random number generator, which is essentially used in every RPG/game that has things like critical hits, and random chances of things happening. for pokemon, the random number generator controls things like when a pokemon will attack you in grass, your chance of a shiny pokemon, chance of critical hits, chance of missing, chance of paralysis.. etc.
there have been RNG programs that people have made to take advantage of Gen 4's RNG (in order to breed/catch shiny pokemon easier), but i don't think Gen 5/black and white have been cracked, yet.