You have pushed your body beyond what it is able to handle, and your ability to act has been reduced. You become exhausted when you have incurred Fatigue in excess of your Fortitude.
Each instance of Fatigue beyond your Fortitude Attribute inflicts Disadvantage 1 on every Check and increases the Tempo of all Actions you take by 1. This means that if your Fatigue is equal to your Fortitude + 3, you have Disadvantage 3 on every Check and the Tempo of all Actions is increased by 3.
If your Fatigue is equal to twice your Fortitude, you have become completely Exhausted and fall unconscious until your Fatigue is reduced below this limit.