In The Witcher 3: Wild Hunt
The Armor statistic describes how much protection Geralt's current armor and/or weapons provide. This statistic can also be boosted with the use of upgrades.
Armor is part of the formula to determine how much damage is taken:
Actual Damage = (Attack Damage - Armor) * (1 - Resistance to Damage Type).
For example, if an a monster attacks Geralt for 1000 damage, and he has 200 armor and 25% monster damage resistance, then first the damage is reduced to 800, and then reduced by 25%, for 600 actual damage taken.
In The Witcher 2: Assassins of Kings
The Armor statistic describes how much protection Geralt's current armor and/or weapons provide. This statistic can also be boosted with the use of potions, trophies and enhancements.