Answer:
in this circuit a capactor charges until the stand off voltage of the ujt is reached and the ujt shuts off causing the capactor to discharge. The time is controlled by the RC time constant of the RC circuit. So what really happens in the ujt conducts and allow the capacator to charge until the ujt shuts off and then the capactor discharges the cycle repaeats itself over and over because as the capactor discharges the ujt will again turn on allowing the capacator to charge