What "Curse" Is the Nazar Talking About?

A Nazar prevents a debuff called Cursed, which you get when specific enemies hit you.

The Cursed debuff prevents you from using your inventory (weapons, potions, anything), so it's highly annoying and, if combined with bad timing, occasionally lethal. It doesn't last that long but that's still many seconds where you are effectively a sitting duck, or if you're smart, a moving target. Luckily it's only applied by a few different creatures (Cursed Skull in the dungeon is one), and most of them have a minor chance to drop that Nazar.

Nazar is the equip item that prevents curse.

Curse is a debuff that prevents the user from using inventory items, the debuff is caused by a Cursed Skull or Cursed Dragon Skull. Both are common in Normal mode, and hard mode if you have not beaten Plantera yet. If you are trying to make the Ankh Charm it is highly recommended that you do so before defeating Plantera.