AFCI stands for "Arc-Fault Circuit Interrupter". Square D circuit breakers designed for AFCI are constructed to detect dangerous low-level arc faults and cut the power when those faults exist. Standard circuit breakers are made to detect overloads and short circuits, but they may not be equipped to break the circuit when an arc fault is detected. AFCI breakers are made to "fill the gap" in safety by detecting all four major potential electrical hazards including ground faults and arc faults.
Square D circuit breakers made to detect arc faults can prevent damage from pre-existing wiring problems and from new damage the wiring system which can happen during remodeling. If you are drilling near a wall socket or breaker box and accidentally damage the wiring, the AFCI square D circuit breakers trip at the first indication of an arc fault. This can save you from a house fire later on due to a condition you might not even realize exists.