The Atmega324p has an 8-channel 10-bit successive approximation ADC. The pins ADC 0 to ADC 7 represent the channels ad set in the ADMUX register. The result is a 10-bit value in the ADCH and ADCL registers. The AREF pin is used as the reference voltage in our case. The refference voltage represents the conversion range of the ADC. There are other options for the reference voltage discussed in the ADC section of the datasheet. The minimum value is represented by GND (0 volts) and the maximum value is represented by the AREF (5 volts).
The prescaler is used to generate an ADC clock frequency. This ADC clock is separate from the system clock we would normally think of when discussing the Atmega324p clock speed. The prescaler starts when the ADEN bit is set high and stops when the ADEN is set low.
See the ADC Registers page for more information about the registers mentioned here.