ADC Frequency: Indicates the ADC clock value based on the ADC prescaler.
ADC Prescaler: Prescales down the ADC clock speed by a specified factor. Prescale factors of 2, 4, 8, 16, 32, 64, and 128 are provided. If, for example, you are running your Arduino board at 16MHz with a prescale factor of 2, the ADC clock is set to 8MHz.
CPU Frequency: Indicates the core clock speed of the target device.