What is "Vref" of ADC in a microcontroller datasheet?
You must provide the reference voltage, which will directly affect the accuracy of your ADC. You could use the supply voltage however that is usually a pretty crude and noisy reference source. 

A reasonable choice would be an external 2.5V reference (either shunt or series type). You can go as low as 1.0V however accuracy will suffer, so it's best to use a relatively high reference voltage.  

The Varef pin has high impedance when the converter is not operating, however it draws 30pC typically per conversion. The maximum current draw is about 65uA so if you want to not affect the accuracy by more than ~0.5 LSB with a 2.5V reference you need a maximum impedance of 5 ohms or so. Even an inexpensive TL431 will be okay at maximum conversion rate. 

If you want tighter tolerance than the 0.5% or so best grade of TL431 then you can use a better reference.