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, each of which blocks only the 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, a shield will begin recovery if it hasn't been damaged in the past 20 seconds. This recovery will be interrupted by taking damage.
If fully depleted, a shield will begin recovery after 50 seconds regardless of whether you've taken the respective damage type recently. This recovery will persist for a few seconds even after you've taken damage.
Sources of Damage Shields