Some skills give you fatigue as you use them.
As your fatigue level gets higher you will not be able to use these skills as you will be too tired.
The best way to reduce your fatigue is to find a bed and to click to rest on it. This will reduce your fatigue back down to zero percent.
Eating food will also reduce your fatigue. Some foods reduce fatigue more than others. Foods such as Pies, pizzas and curry are among the best foods for reducing fatigue.