You've been exposed to Harold's mutation and your skin is now as hard as tree bark. As a result, you've gained a permanent +5% to your Damage Resistance.
The Fallout 3 Official Game Guide gives this perk's benefit as being +1 Endurance and +10 Damage Resistance. This information was based on an early build of the game, and does not reflect the final version of the perk in the released Fallout 3. The final version of this perk only gives +5 Damage Resistance.