The Number of Power Line Cycles (NPLC) indicates how long an input signal is integrated to obtain a single measurement. Noise introduced from the power line tends to be periodic. If the A-to-D converter acquires the input signal for the same amount of time as the 60Hz (or 50Hz) power input, then the effect of the periodic noise can be integrated and cancelled. Generally speaking, the longer a signal is integrated by the A-to-D converter, the more accurate the resulting reading will be.