The phrase "stopwatch function" refers to a feature on a device or tool that allows you to measure and track the amount of time passed. It is like a digital stopwatch that can start, stop, and reset the timer. Full definition
If you download your track plan from a USB device, you can even use the automatic stopwatch function to track your progress, lap by lap. (topspeed.com)
The new instrument cluster features a large multi-function digital display (single or triple - window, depending on model), which displays information such as radio station display, PRND321 gear selected indicator, trip computer with stopwatch function, service reminders and a help facility. (en.wikipedia.org)
It only read heart rate and time, including stopwatch functions, and it doesn't connect to its own online system where you can analyze your fitness trends over time, although you can sync the data to some other apps. (pcmag.com)