Crew members can become Fatigued when they trigger certain Crew Effects. Fatigue wears off at the start of each Player Phase, so this effectively limits each Crew member to triggering at most one Crew Effect per turn.
Fatigue only affects the ability of a Crew member to trigger Crew Effects; it doesn't prevent the Crew member from unlocking actions or bonuses for example by manning Nodes or going to spaces on the Port Board.