Strata or Stratum are terms used to describe the distance a particular device is from a time source. The further away it is the higher the stratum, and the less accurate it's time synchronisation will be.
Stratum levels range from 0-15 and dictate how many "hops" the device is from the reference source.
Stratum 0 describes a time source like an internal atomic clock or GNSS signal.
Stratum 1 is a device directly synced to the source, outputting an NTP signal to a stratum 2 device. Tekron clocks are stratum 1 devices.
Stratum 2 is a device connected to a Stratum 1 device, Stratum 3 is connected to a Stratum 2 device, and so on.