Frequency
Frequency is normally expressed in hertz (Hz).
When a signal cycles from high to low then high again 8 times per second it is expressed as 8 Hertz.
ie: 1 Hz = 1 cycle per second, 40 Hz = 40 cycles per second.
Setting the correct PWM output frequency will depend on the device being controlled and can only be obtained experimentally by examining the behaviour of the device.