The term chilled brooding on bees?

Bee brood -- eggs and developing larvae -- have to be kept warm at around 35°C (95°F). The bees do this by covering the brood cells with their bodies and vibrating their flight muscles without moving their wings, thus generating heat.
If, for any reason, there are not enough bees in the hive to cover the brood fully, the brood that is not covered will get too cold and will die. This is called 'chilled brood'.