A chronograph is basically a complication used to measure elapsed time. Generally chronographs have two pushers to control its function including one for start/stop and another to reset. A monopusher chronograph on the other hand has a single pusher that goes in the order of start, stop, reset when pushed consecutively.
Chronographs are generally one of the more common complications on mechanical watches. Back in the day, the ability to measure elapsed time came in handy for various purposes, some of which are replicated on watches found today. The most famous of which is the tachymeter which can be used to calculate speed by measuring how long it takes for an object to travel a specific distance. Then, there is the pulsometer with a scale designed to show a person’s bpm based on how long it takes to count 30 pulsations. Another good example is a telemeter scale that will calculate distance based on the time taken from when something is seen to when it is heard.