If a scientific study were being done on employees of a company to see if changing their hours would affect productivity, the Hawthorne effect says that their productivity could change just because the study was being done and not because of the change in work hours.
At its most basic level, the Hawthorne effect describes a phenomenon where people modify their behavior not in favor or disagreement of a change but rather just because a change was made. One study was performed at a factory to discover if a change in lighting would affect productivity. Productivity did increase but only because of the increased attention due to the study. As soon as the study had ended, productivity decreased again. In this example, the change in productivity had nothing to do with the lighting but rather the existence of the study in the first place.