Stratum 0 refers to the most fundamental layer in the Network Time Protocol (NTP) hierarchy. It consists of high-precision timekeeping devices, such as atomic clocks or GPS clocks, that provide the accurate time reference. These devices are essential for maintaining synchronization across various systems. The information from Stratum 0 is not directly accessible over the network. Instead, it feeds time data to Stratum 1 servers, which then distribute the time to lower strata. This hierarchical structure ensures that accurate time is maintained throughout the network, benefiting applications that rely on precise timing.