A monopusher is a specific kind of chronograph, where only one button is used to start the timing, to stop it, and to reset the chrono hand to the “0” position (normally 12 o’clock).
For many years, chronograph complications on pocket watches (and later, wrist watches) used a single operating pusher to perform three tasks. In order for a single pusher to do three things, it merely cycled through starting, stopping, and resetting the chronograph function. That meant that a user could not pause timing something. If they stopped timing an event, then they needed to reset the chronograph before resuming their timing of that event. Thus, you can see that monopusher chronographs are more limited in their functionality than two-pusher chronographs. That is because in the modern standard two-pusher system, one pusher can be used to start and stop the chronograph multiple times, while a separate pusher is used to reset it.