The power, timer, ready, and oil light are all indicators when the unit activates them. The power light is on when the deep fryer has reached its desired temperature, and it's working correctly. The timer light goes off when the unit's digital timer runs out of time, and then the ready light will come on to let you know that your food is prepared to eat! Lastly, the oil light comes on once you've added oil to the unit, so you know when the oil is heated adequately for frying.