The Buzzer module plays a 4kHz tone. The volume of the buzzer can be adjusted by changing the amount of power going through it. The plus and minus signs indicate the direction in which it operates.
The buzzer module uses a 4kHz fixed frequency, piezoelectric DC buzzer which operates in one direction. An internal oscillator that produces a 4kHz alternating current creates the pitch. As current flows in either direction across the piezoelectric material, it changes shape and creates the sound. An increased voltage will increase the amount of deformation of the piezoelectric material, increasing the amplitude of the sound wave being produced, and the volume of the sound we hear.