There are three headings shown during radio duty cycle simulation in cooja: radio_on, radio_rx and radio_tx. The meaning of radio_rx and radio_tx are obvious. What is meant by radio_on option?
It's the time when the radio chip hardware is turned on, i.e. is either in ready-to-receive state, Rx'ing or Tx'ing.
And by the way, when the radio chip is on and neither transmitting nor receiving, the energy consumption is almost the same as in receive mode. The energy gets spent on keeping the receive machinery active and continuously sampling the medium to detect start of a packet.