Moral Luck is the idea that even in an action that results in some kind of horrible accident or consequence, there is an element of luck that either placed the person in the course of events that lead to that accident or consequence, or that prevented it from happening.
A frequently used example is that of a person running a red light. In one instance, nothing happens. in the second, a person happens to be crossing the street and the driver cannot avoid them and kills them.
In both cases, the immoral action was the same, but the first driver had good moral luck while the second had bad moral luck.