Ok, thanks. But @DVDdoug has clearly pointed out that when he said 'transistor (or MOSFET) driver'.
What I didn't understand indeed, is the meaning of 'driver' in electronics. Because I know about a different meaning in programming.
Conceptually it is more or less the same thing, a driver program is a code piece that knows how to talk to something (like an ethernet board or a serial port) and presents an API to the programmer.
In electronics a driver (as in "LED driver") is a component or circuit that knows how to talk to something and presents you with a way to tell him what you want to do.
In this case the transistor is a driver because it gives you an interface (the base pin) that you just have to switch HIGH or LOW, when you switch that pin HIGH the transistor will switch on and current will flow trough the buzzer, when you switch that pin LOW the transistor will switch off.
So, you don't need to worry if your Mega sends 3.3 or 5 V, the driver circuit will provide the correct voltage to the device. Of course that in this case of a very simple driver you'll have to build it yourself making sure that it will provide the voltage that your device need, so, in Electronics, the final user will have to know more about the device it is working with than a programmer that's only using an API.