difference between vdd and vref in adc

Hi,
please expain purpose and differnce between reference voltage(vref) and supply voltage (vdd) in adc.im not understanding why v should apply two voltages to adc.

Vdd is the voltage for the MCU and Vref is the reference voltage for ADC. If you are inputting 0v to 5v to any adc pin like RA0 in PIC MCU then if you set the reference voltage bits to 0 in the register bits for adc reference voltage then Vref+ will be 5V and Vref- will be 0v. So, your adc will give a value of 0 to 1023 for 0v to 5v. What if you are inputting only 0v to 1v for the adc pin? Then if your Vref+ and Vref- are 0v and 5v, then you will get 0 adc value for 0v and 1023 adc value for 5v, but for 1v your adc value will be 204.6. So, your adc range for 0v to 1v will be 0 to 204.6 adc value and your adc resoulution will be 0.0048875855327468. If your Vref+ is 1v and Vref- is 0v you get 0 to 1023 adc value for 0v to 1v so, your adc resolution will be 0.0009775171065493646

0.0048875855327468 * 205 = 1
0.0009775171065493646 * 1023 = 1

VDD is for Digital circuitry and vref is a reference supply for analog front end to convert analog signal to digital.