'Strata' refer to the hierarchical levels used by Network Time Protocol (NTP) to disseminate time information over a network.
Stratum 0 refers to a device that is synchronised directly to the chosen time source (e.g. GPS, MSF-60, DCF-77, WWV, etc) and therefore has no discernible delay from it.
Stratum 1 refers to a device receives time information directly from the Stratum 0 device via serial (RS232, RS422, etc) or even timecode (IRIG-B, AFNOR, NASA codes, etc). The connection can be external, but some Stratum 1 timeservers have the Stratum 0 device built inside them for convenience. The time delay accuracy is generally less than 1 millisecond.
Stratum 2 refers to devices that are connected to the Stratum 1 timeserver via a network. For example, computers on a LAN are Stratum 2 devices. Each Stratum 2 device can be linked this way to more than one Stratum 1 time server to optimise the information it receives. The delay accuracy can usually range from 10 to 100 milliseconds.
Stratum 3 is the next level connected via a network path, and so on.
All our time and frequency standards and modular timing systems are Stratum 1 NTP time servers.