Acid is one of the 7 elemental damage types.

Acid damage is the first elemental spell that every player starts with. It is generally the preferred type of damage against animals until the mage reaches level 125 and can learn the racial damage types, after which it is rarely used anymore except in PvP.

To increase your chances of resisting an Acid elemental spell, you have to wear jewelry that raises your Prime Roots resistance.

Lumpers, Zerx and Kinreys are all capable of spitting acid at their enemies, though these attacks are physical and not magical in nature. That means Acid protection will protect you against them, but Prime Roots resistance will not help you resist them.