Reciprocity failure is something that occurs during long exposures, especially relevant to film photography but can also affect digital sensors to some degree. In normal conditions, there's a direct relationship between the amount of light and the exposure time to get the correct exposure. However, with long exposures, this relationship breaks down — the film or sensor becomes less sensitive to light over time. So, following the standard rules for exposure, the image would end up underexposed. To correct for reciprocity failure, one needs to adjust the exposure time longer than what would be normally calculated to ensure a correct exposure. This can often take a bit of trial and error to get the correct timing for ones camera.