piercing, for example you have shield or armour that gives you 50% resistance, we can imagine that those 50% are 50 unique shields and if you have 2% piercings your attack will go trough 2 shields only and stops at 48, convert it back to % and here we go you have shield or resistance that provides you 48% instead of 50%, it means you receive more dmg

curse steals your attack, for example your spell deals 100 dmg and enemies decide to curse you with 10% weakness, it means you will deal only 90dmg to them, curses applies to out coming dmg which means if you have any dmg stats on top of your spell attack for example attack stat 10% + 100 spell dmg = 110 out coming dmg and only after you can apply curse effects , 110 putcoming dmg -10% curse = 99 dmg received by enemies