DR means Damage Reduction.
Damage Reduction (DR) is a stat that reduces damage received by a percent from all attacks (both physical and magical- even skills). It can be only upgraded through the DEX stat. Having 20 DR means 20% damage reduction. Thus, it reduces all damage, except from negative effects (i.e. poison, lasting fire and bleeding). On 90 DEX, characters have 30% Damage Reduction.