Fortune is a skill level that grants individuals good luck in their daily lives. The definition of "luck" is very vague and philosophical so it's hard to determine exactly how the skill level functions. One can say that those who possess this skill tend to get favored by circumstances in an unpredictable way. Typically, this happens without the user's control and even against their will, and events just so happen "randomly" go to their advantage.
Those with higher Fortune skills have events around them unwillingly molded in even more absurd ways. However, unless one was the Hero or a Balance Breaker, luck is a very unreliable tool and no matter how lucky one is, there are certain occassions in which a scenario can't be avoided, rendering Luck useless in extreme circumstances (though one would often naturally avoid those altogether). It also doesn't take place every time in every case, and is completely unpredictable, not to mention that the results it can unfold while always positive for the user, may not necessarily be what one desired.
This skill level is one of the few to have inverse values, in which case it is called Misfortune. Those with this skill level are always going through hard times, and higher levels have disaster striking them time and again, to the point it looks like the universe hates them. For some reason, clumsy girls with glasses are the ones that tend to have this skill level.