Stratum is a term that means different things depending on the context.  In the world of NTP, stratum is defined in RFC 1305.  NTP uses a hierarchical structure in which Stratum 0 is the reference clock, linked via a time signal, to a reliable source of UTC.  Stratum 1 is the time server with a direct link to the reference clock.  Stratum 2 is a client that receives time over a network connection from a Stratum 1 clock.  Stratum 3 is a client that receives time from a Stratum 2 clock.  And so on, up to Stratum 15.  For more details on strata in the NTP world, click here.