Is there two different elemental resistance stats or am I missing something. I crafted the Barroth armor to help me take down Lagiacrus with its Thunder defense. This defense is separate from the resistance skill (which requires 10 points to activate). But what effect does it has? From what I could find in the Wiki it gives me 5% less damage from Thunder attack. Is this correct or have I gotten it wrong?

You mean the Barroth armour on this page?

http://monsterhunter.wikia.com/wiki/Barroth_Armor_(Blade)

I don't know what you mean by 2 different elemental stats but that armour clearly states with the full set you will take 15% less thunder damage.

Yes. There are stats found on gear, like attack and defense and elemental defense. Then there are skills which activate when you have the right armor/gems equipped. These skills can provide you with a variety of effects, including additional attack, defense, and elemental defense.