A stunt is a special trait your character has that changes the way a skill works for you. Stunts indicate some special, privileged way a character uses a skill that is unique to whoever has that stunt.
Stunts have to make since though to your characters backstory just like anything else. They can also change like most things as your character changes. As in you could decide a stunt no longer makes since with who your character has become and you may want to swap it for another stunt or exchange it for a fresh at the next Milestone.
Most stunts just give you a +2 to a skill, but they can also combine two skills in special circumstances where it would make since. They can do far more then just that. The book has some great examples that you can use for the game if you don’t want to create your own. Keep in mind though that the more powerful the stunt is the more likely I will require you to use a fate point to use it and if it’s way too powerful, I will probably not allow it at all. Think outside the box, but within reason.