Video games are nothing more than computer programs, and they're limited by the systems they run on. One particularly strict limitation is the fact that computers can't handle numbers of arbitrary size with any kind of efficiency. Since efficiency is normally pretty important for video games, this places some hard limits on the range of values they can work with. Since computers work in binary, that size has to be a certain number of bits (binary digits - a zero or a one). Numbers stored in a computer's memory are called variables. And so on.