Plain English Meaning
Extra fuses kept in the aircraft so that if one burns out and breaks an electrical circuit, the pilot can swap in a new one and restore the affected equipment.
Full Definition
Replacement fuses of the correct type and amperage carried on board an aircraft so that a blown fuse in an electrical circuit can be replaced in flight or before further flight. For aircraft using fuses (rather than circuit breakers) to protect electrical circuits, federal regulations require that one spare of each type and rating, or 50% of the number of each type installed (whichever is greater), be carried on the aircraft.
Context Anchor
Seen during preflight equipment checks, especially when reviewing required equipment for night flying in an airplane that uses fuses.