General

Physical Defense Rate/Resistance, like said by other people... On equips potentials it takes away a certain percent of that...

Physical Damage Resistance.
It means that if the item has 30% PDR reduce on the monster, enemys PDR is lowered by 30%. Normal monsters have PDR of 10% so with 30% reduce they only have 7% PDR left when you attack, which means you deal 93% of your normal damage (= range * skill power).
Most bosses have 30% PDR, so they'll have 21% left.
Zakum and HT have 50%, so they have 35% left.
PB has 70%, so it's lowered to 49%.

When compared to boss damage 30%, PDR reduce is better at PB. They both deal same damage at Zak and HT. At rest of the bosses, boss damage wins. And on normal monsters, PDR is better, since Boss damage has no effect (obviously).

A lot of people's minds says, "Who cares about defense, I NEED MORE ATTACK!" Well this benefits with your attack It's a good thing nexon made this type of potential

Yep, PDR is the Physical Defense Rate of bosses. The % is how much damage is reduced from your attacks unless you have the means to ignore it (skills, potentials, whatever). Weapons can come with the ability to ignore PDRate% from potentials. xD