Fatigue
Fatigue simply stands for how tired you are. All skills contribute to fatigue. Every time you mine an ore, catch a fish, or fight a monster, your fatigue percentage increases. Once you are very fatigued, or in other words, your fatigue percentage is very high, you must rest in order to replenish yourself before you can do anything else.
Find any bed and click on it. You will begin to sleep and your fatigue percentage will drop to 0%. To wake up again, you must type the word you see. Sometimes the word is unreadable due to the type of font used (this is to protect against automated programs that type for you). You will be given an option to get a new word displayed if that is the case.
Another good way to heal your fatigue is to sleep on a sleeping-bag. The advantage of a sleeping-bag is that it is portable, unlike a bed. However, it works the same way. You click the sleeping-bag and then you have to type the word displayed to wake up again. Sleeping-bags can be purchased at General Stores.