What does different stratum values in ntp output means ?

Issue

• What does different stratum values in below output means ?

Feb  7 17:03:54 hostname ntpd[3772]: synchronized to 10.255.255.3, stratum 8
Feb 10 00:25:01 hostname ntpd[3772]: no servers reachable
Feb 10 00:42:20 hostname ntpd[3772]: synchronized to 10.255.255.3, stratum 3
Feb 10 22:53:55 hostname ntpd[3772]: no servers reachable
Feb 11 00:19:21 hostname ntpd[3772]: synchronized to 10.255.255.3, stratum 3
Feb 13 16:55:33 hostname ntpd[3772]: no servers reachable
Feb 13 17:46:45 hostname ntpd[3772]: synchronized to 10.255.255.3, stratum 4
Feb 13 18:05:57 hostname ntpd[3772]: time reset +128.609837 s
Feb 13 18:06:32 hostname ntpd[3772]: synchronized to 10.255.255.3, stratum 4
Feb 14 01:09:39 hostname ntpd[3772]: no servers reachable
Feb 14 01:44:05 hostname ntpd[3772]: synchronized to 10.255.255.3, stratum 4
Feb 14 03:09:07 hostname ntpd[3772]: no servers reachable
Feb 14 08:16:27 hostname ntpd[3772]: synchronized to 10.255.255.3, stratum 4
Feb 14 11:24:21 hostname ntpd[3772]: no servers reachable
Feb 14 11:24:37 hostname ntpd[3772]: synchronized to 10.255.255.3, stratum 2

• We have Redhat Enterprise Linux 5 & 6 servers and we are planing to change the NTP stratum value online so please check and provide the steps for it.