Put simply, a chronograph is a stopwatch, often found on motor-racing inspired watches. There are many variations of this complication which can measure several fractions of time, such as milliseconds, minutes, hours etc. It is important to make sure the chronograph is turned off when not in use, as it can drain the battery in quartz watches, or sap the power reserve and potentially damage the movement in mechanical watches. More information on caring for your watch can be found here.