Motor Shield frequency

[fitzgeralds]

This is in reference to the Motor shield.
There is something that I do not understand.
In the DCMotorTest Sketch, it states:
“AFMS.begin(); // create with the default frequency 1.6KHz
//AFMS.begin(1000); // OR with a different frequency, say 1KHz”
Could someone explain to me, what is this frequency, and why do I need it?
The reason I ask is because in thought that PWM was a frequency. Im confused!
Or is this the PWM carried on a Frequency? Is this that frequency?

[adafruit_support_bill]

PWM is "Pulse Width Modulaton". It modifies the width of the pulse, providing a variable duty-cycle of 0%-100%
The frequency at which the pulses are generated can be set in the begin() statement. The maximum PWM frequency for the shield is 1.6KHz.

[fitzgeralds]

Thank you