An arc fault is a high-power discharge of electricity between two or more conductors. This discharge generates heat, which can break the wire's insulation and trigger an electrical fire.
Arc Fault protection is a wiring protection and Fire prevention required by the National Electrical Code.
Either a special type of receptacle or circuit breaker that can sense certain types of arcing that are known to cause fire. In most parts of a home, if you change out even one receptacle the code requires Arc Fault protection. Also, in many cases, in a home, if adding or replacing wiring Arc Fault protection is required.