@ggganz - naterator isn't quite right: that's mixed seed.
All "luck" in Civ4 is handled by the RNG (Random Number Generator). a mathematical device which produces a series of numbers in which, theoretically, there is a nearly equal chance of occurrence of any number. This calculation requires a number to use as the basis for calculating the series, and this starting number is known as the seed. The same seed will always generate the same "random" sequence, which is why there is the option to use a new seed at reload time if you don't want to get exactly the same result from an action after reloading as you got before the reload (which probably was not what you wanted, and hence the desire to reload).