It changes a buff that is applied to every vanilla dragon and most likely modded dragons. What it does...
Physical Damage Reduction: 80%
Spell Absorb: 80%
Resist Magic: 85%
Magicka Regen Rate: 30%
Armor: 50 Points
Fortify Stamina: 100
Health: 825
Cannot regenerate health
Keep in mind this is in addition to any stats a dragon has so it can lead to some Dragons becoming immune to some elements.
As well as that the additional health the dragons receive will NOT be shown on the bar due to how Skyrim handles the fortify health buff on enemies. You are doing damage, just can't see it yet...