Hello Sumit
Root dispersion is a rough estimate of of how much the local time has drifted away from some reference time. This reference time is often referred to as the root source. Root dispersion occurs when there is some malfunction in the time source. A disruption in service or a bad network connection can cause a steady increase in root dispersion. For Cisco IOS devices, any root dispersion in excess of one second will cause an NTP association to fail.
Root dispersion is also included in NTP updates so that NTP clients receiving them can choose to use them, or ignore them in favour of another NTP server.
In any case, root dispersion is not something that you can deal with directly from the local device. If you are experiencing problems with root dispersion, the best thing to do is to choose another time source. If the time source you are using is administrated by you, then you should troubleshoot that time source and verify that it is functioning correctly.
Root dispersion will also be seen in special cases where Cisco IOS devices are attempting to gain NTP information from Windows Active Directory. This is further detailed in the following Cisco documentation:
I hope this has been helpful!
Laz