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 elemental magic and can learn the racial damage types (fire, poison, shockwave and electric). After that Acid is generally rarely used anymore except in PvP.
Most animals have no protection against Acid magic; exceptions are Arana, Lumper, Najab, Shalah, Tyrancha, and Zerx. By contrast, all intelligent plants and all kitin have protection against Acid magic. Kiban, Kipee, Kincher and Kizarak are even entirely immune to Acid spells. All creatures in the Prime Roots also have significant protection against Acid.
Protection from Acid
All player characters have a default 10% protection from Acid damage. You can protect yourself against Acid damage further by wearing jewelry that gives additional Acid protection. You can have at maximum 70% protection.
You can also increase your chances of resisting an Acid elemental spell by wearing jewelry that raises your Prime Roots resistance.
Non-magical Acid damage
Lumpers, Tyrancha, Zerx and Kinreys are all capable of spitting acid at their enemies, but 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.
Many bandits and tribes guards also have random elemental damage added to their melee and ranged attacks, including Acid. If you're hit with Acid damage that way, Acid protection will decrease that damage, but Prime Roots resistance will not help you resist it.