Hormesis is the way in which a minor amount of damage can be beneficial to an organism: it awakens cellular repair and maintenance mechanisms that fix that damage and then go on to more energetically fix other damage for an extended period of time. So the net result is a more robust, better maintained individual with a longer life expectancy. There are hormetic components to the benefits of exercise, calorie restriction, mild heat stress, and so on.
Mild radiation damage is one of the ways in which hormesis can be achieved, as examined in this review paper:
This paper assesses the capacity of ionizing radiation to extend the lifespans of experimental insect models based on the peer-reviewed literature. Ionizing radiation biphasically affects the lifespans of adult males and females for a broad range of insect models with high doses reducing lifespan whereas lower doses can enhance lifespan, typically in the 20-60% range.
The average adult insect lifespan can be increased when ionizing radiation exposure is administered during early developmental stages or during the adult stage. The effective dose inducing the average adult insect lifespan enhancement may vary considerably depending upon which life stage is exposed. Recent findings have identified specific genes affecting anti-oxidant defenses, DNA repair, apoptosis and heat shock proteins as well as several cell signaling pathways that mediate the longevity enhancing hormetic response.