[Discuss-gnuradio] What is the purpose of argument 'OK' in rx_callback

[bharadwaj desikan]

Hi

Can anyone out there, Kindly do explain the condition during which the argument 'ok' of rx_call back function return false.ie 
in the below code n_rcvd will be increamented while n_right will not .

Also if  disturbance in transmission will this affect this during reception
  
  def rx_callback(ok, payload):
        global n_rcvd, n_right
        n_rcvd += 1
        if ok:
            n_right += 1

Thanks you in advance.