Chronographs can be confusing at first glance, both in name and functionality. The word chronograph sounds similar to chronometer, and the subdials can be bewildering if you don’t know what they’re for (short answer: egg timer).
Chronographs measure periods of time independently from the time of day, like a stopwatch. Chronometers, on the other hand, refer to a high overall precision and accuracy of a watch. Chronographs that are chronometers are common, but the two terms should not be confused with each other. Some notable chronographs are the A. Lange & Söhne Datograph, the Rolex Daytona, the F.P. Journe Centigraphe, and the Omega Speedmaster.
Now that the meaning of a chronograph is understood, the purpose of the subdials start to become clear. The typical chronograph features a subdial for running seconds, chronograph minutes (30 minutes measured), and chronograph hours (12 hours measured). The chronograph seconds hand is typically on the top of the regular hours and minutes hands stack. The chronograph is started, stopped, and reset by using the two pushers near (one above and one below) the crown. There are notable exceptions and additional complications to these features, but this is how the common chronograph is designed. It’s good to be weary of running the chronograph constantly, as it can put needless wear on the movement and drain the power reserve.
Source: Shuck The Oyster
A chronograph can be used for more than just timing eggs, however. It can also measure average speed over a distance or pulsations using a tachymeter scale or pulsometer scale, respectively. A telemeter scale can also be used to measure the distance between a visual cue and sound, but even with all of these cool things you can measure with a chronograph; the average owner will probably just use it as an egg timer.
Time of Horology 