In Fate, aspects do two major things: they tell you what’s important about the game, and they help you decide when to use the mechanics.
Your collection of game and character aspects tell you what you need to focus on during your game. Think of them as a message from yourself to yourself, a set of flags waving you towards the path with the most fun.
GMs, when you make scenarios for Fate, you’re going to use those aspects, and the connections between aspects, to generate the problems your PCs are going to solve. Players, your aspects are the reason why your PC stands out from every other character who might have similar skills—lots of Fate characters might have a high Fight skill, but only Landon is a Disciple of the Ivory Shroud. When his path as a disciple comes into play, or the Ivory Shroud takes action, it gives the game a personal touch that it wouldn’t have had otherwise.
The game aspects do something similar on a larger scale—they tell us why to care about playing this particular game in the first place, what makes it concrete and compelling to us. Everyone can all say, “Oh, we like space opera games,” but until the group drills down to the specifics of a universe where people will do Anything for Survival, and where The Empire is Everywhere, they don’t really have anything to attach their interest to.
Situation aspects make the moment-to-moment interactions of play interesting by adding color and depth to what might otherwise be a boring scene. A fight in a tavern is generic by nature—it could be any tavern, anywhere. But when you add the aspect Huge Bronze Devil Statue to the scene, and people bring it into play, it becomes “that fight we were in at the Bronze Devil, when I smashed that guy’s head into the statue.” The unique details add interest and investment.
Because aspects tell us what’s important, they also tell us when it’s most appropriate to use the mechanics to deal with a situation, rather than just letting people decide what happens just by describing what they do.
GMs, this comes up for you most often when you’re trying to figure out whether to require a player to roll dice. If a player says, “I climb this ladder and grab the idol,” and there’s nothing special about the ladder or the idol, then there’s no real reason to require an overcome action to grab it. But if the situation aspects tell you that the ladder is a Rotting Rope Ladder and the idol is Protected by the Wrath of the Gods, then you suddenly have an element of pressure and risk that makes it worth going to the dice for.
Players, this comes up for you most often when invoking your aspects and considering compels. Your aspects highlight what makes your character an individual, and you want to play that up, right? So when the opportunity comes up to make your character more awesome by invoking, go for it! When you see an opportunity to influence the story by suggesting a compel for your character, do it! The game will be much richer for it as a whole.