What is Analog and Digital supply voltage
Your digital supply can be quite noisy and you will get away with that in many digital applications but, that noisy supply would not be good enough for an analogue to digital convertor in most cases. Also, your ADC may have the option of using the analogue supply as its analogue reference input - this would then require the analogue supply to be much more stable than the digital supply in order to get best accuracy from the ADC when doing conversions. So in summary: -


Digital supplies can be quite noisy and still work
Noisy analogue supplies can cause problems
A digital supply that is approximately correct in value (i.e. 5V +/- 0.5V) will be OK for most digital circuits
An analogue supply used as the reference to an ADC would need to remain much more stable (e.g. for a 10 bit ADC, you'd want the reference voltage to be (say) 5V +/- 5mV or better.



  How can I use these pins during designing of supply section.


If your digital supply is quite clean and you are not going to use the ADC reference tied to this supply then in most cases on MCU designs you can connect them together but it is application dependent and you need to provide more details.