Your fatigue represents how exhausted you are. The effective maximum HP of your Head is reduced by the value of your fatigue, meaning you can go unconscious if you have too much. In addition, fatigue gives a disadvantage to all checks made (by the amount of your current fatigue). You typically get fatigue from not sleeping, extreme temperatures, not eating or drinking enough or marching extreme distances without rest. There is no upper limit to fatigue; in extreme conditions, it may kill you.
A full rest (8 hours of sleep or inactivity) removes one level of fatigue
Some proficiencies will also give you fatigue if you use them for prolonged durations. See the Proficiency list for more details.