Darkvision reduces the darkness penalties to Attack, Defence, and Precision depending on the number next to the icon. Darkvision (50) reduces it by half while (100) eliminates it entirely. Undead units always have perfect darkvision.
Different kinds of darkness incur different stat penalties:
- Night/Cave/Deep Sea/Solar Eclipse/Theft of the Sun: -3
- Utterdark/Darkness: -6
- Blind: -9
Darkvision does not help you if you have been blinded, and is unnecessary if your unit does not use eyes to orient itself.