Damage Shields are pools of temporary health that take damage before your Health is reduced. There are two types of shields: Bullet Shields and Spirit Shields which can only block their respective damage types. Damage Resistances do not apply to shields. Once damaged or fully depleted, they will fully recover after some time.
Recovery
If not fully depleted, shields will begin recovery if they haven't been damaged in the past 20 seconds. This recovery will be interrupted by taking damage.
If fully depleted, shields will begin recovery after 60 seconds regardless of if you've taken its respective damage type recently. This recovery will persist for a few seconds even after you've taken damage.
Sources of Damage Shields