Dwarven Shield Mastery:
You gain an additional bonus to reduce the damage you suffer when blocking. (+2, +4, +6 for levels I, II and III respectively)
Shield Mastery:
Increases the amount of damage you can absorb when defending with a shield.
Prerequisite: Shield Proficiency: General (+3 bonus)
Improved Shield Mastery:
Increases the amount of damage you can absorb when defending with a shield.
Prerequisite: Base Attack Bonus +8, Shield Mastery (+3 additional bonus)
As you might know, DDO has implemented and Active and Passive Danage Reduction system.
Adamantine, the Barbarian's DR, Stoneskin etc are all considered passive. However, there is a category of Active DR and involve the feats
Shield Mastery and
Improved Shield Mastery and the Dwarven Enhancement,
Dwarven Shield Mastery.
Two-Weapon Blocking also falls under this category.
Also, shields, according to their category, impose a Damage Reduction when actively blocking. Bucklers and Light Shields reduce the damage by 1, Heavy Shields by 2 while the Tower Shield 4.
Finally the base formula to find out what the DR with a shield on is, would be:
Base active DR (with shield) = (BAB / 2) + Shield AC bonus + 2
On this one, we shall add the bonuses from feats, enhancements and the Shield's magical bonus (+1 to +5).
A level 12 dwarven fighter holding a +5 tower shield and actively blocking would have a DR of:
Active DR = (12/2)+6+3+3+(4+5)+2= 6+6+3+3+9+2= 29
I do hope that I have answered your question, Captain. Feel free to ask for clarifications and I shall update my post.