Stratum tells us the quality of the reference clock:
The Stratum model is a representation of the hierarchy of time servers in a network, where the Stratum level (0-15) indicates the device’s distance to the reference clock.

Stratum 0 means a device is directly connected to e.g., a GPS antenna. Stratum 0 devices cannot distribute time over a network directly, though, hence they must be linked to a Stratum 1 time server that will distribute time to Stratum 2 servers or clients, and so on. The higher the Stratum number, the more the timing accuracy and stability degrades.

The NTP protocol does not allow clients to accept time from a Stratum 15 device, hence Stratum 15 is the lowest NTP Stratum.

A group of NTP servers at the same Stratum level (Stratum 2, for example) are considered NTP Peers to each other. NTP Servers at a higher Stratum level, on the other hand, are referred to as NTP Servers.