Editing Unwritten: System Reference Document/Game Basics (section)

===Skills and Rolls===

Skills determine what a character can do and how well they can do it. Each player has a number of skills, each with a rating indicating how good they are at it. Everything else that a character does is considered "Mediocre (0)".

When you try to do something, you roll the four Fate dice, add up the pluses and subtract the minuses, then add an appropriate skill. 

All else being equal, if the result is positive then you succeed. If the result is high, you do really well. If the value is negative, then things don’t work out for you. If the dice total up to zero, then you don’t quite do it, but you get close or make progress.

Of course, nothing is ever completely equal. Other factors will shift the result up (add to the result) or down (subtract from the result).