The sendInterval control how frequently updates are sent for this script.
If sendInterval is zero, updates will be sent at the end of the frame when dirty bits are set for that script. Note that setting the value of a SyncVar will automatically set dirty bits.
If sendInterval is non-zero, updates are deferred until sendInterval seconds have passed since the last update for that script. So it can be used as a throttle in cases where the Sync value is changing constantly on the server, but you don't want it to be updated every frame.
The default sendInterval for scripts is 0.1f seconds.
The send interval can also be customized by implementing the virtual function GetNetworkSendInterval() on NetworkBehaviour.