Welcome to AudioCircle, samratraj!
The answer in one word is "jitter"
Jitter is the timing variation between digital samples. A CD streams 44,100 samples per second and they should be exactly 1/44,100th of a second apart. When this interval fluctuates longer and shorter it's called jitter. Most older DACs rely on the timing of the incoming digital signal for their own timing, meaning they make no extra effort to reclock the samples to reduce jitter. This is 95% of DACs in the world. But now that jitter "is a thing" audiophile DACs and transports do make efforts of varying success to reduce jitter.
I used Sonos ZP-80 as my main digital source for many years. It is very high jitter. I recently upgraded to the cheapest Aurender N100 and the difference was impressive. A friend sold the N100 to me after he upgraded to N20 and experienced a noticeable improvement over N100.
