This is the way I see it.
The driving signal is specified as a square wave on the data sheet and it is used to turn the MOSFET on or off like a switch. The spec sheet says the buzzer has a resistance of 12 ohms with a maximum current rating of 100 MA. With the series resistor of 100 ohms and the buzzer resistance of 12 ohms, when the MOSFET is on, you have 112 ohms across a 3.3 VDC supply which gives you about 29 MA. That gives about a .35 VDC drop across the buzzer and about a 2.9 VDC drop across the resistor.
To drive the buzzer with the maximum current you would need a 21 ohm resistor, in series with the 12 ohm buzzer, or 33 ohms across the 3.3 VDV supply to give you 100 MA. Apparently this would produce far to loud a tone for this application so the circuit design drives the buzzer at less than a third of the maximum current.