Everfrost resistance is a new statistic added with the release of Module 10:Storm King's Thunder. It operates similarly to black ice resistance, previously introduced in module 3. Everfrost Resistance works the same way that defense would against Everfrost damage, but provides no benefits against non-everfrost damage.
Most of the epic items introduced in module 10 and module 10.5 have everfrost resistance as a stat.
Everfrost resistance can also be gained through specific consumable potions, and through armor reinforcement kits that are purchasable through the Storm King Thunder's campaign store.