Film loses its sensitivity to light the longer it is exposed to light. When shooting under normal conditions, this doesn't really matter. But when shooting in low-light or any scenario with a long exposure, reciprocity may need to be taken into account in order to get a proper exposure and avoid underexposure.
Finding the correct exposure time is easy. The correct exposure (c) is found by setting the light meter's speed reading (s) to the power of the film stock's reciprocity (p) factor. c=s^p
Quick disclaimer: color reversal or slide film(s) reciprocity isn't calculated here yet, due to having a greater tolerance for reciprocity.