There are several ways to interpret the question, and so there are several answers. That makes it more fun.
It is not good to get acid in your eye. It hurts. The acid changes the shape of the proteins in your eye and skin, so they don’t work the way they normally do. Your body responds to this by sending pain signals to your brain, so you stop putting acid in your eye.
In your stomach, acid is produced by your body to help you digest your food, and to kill microorganisms that might make you sick. It changes the protein called pepsin into the right shape to help break down the proteins in your food. Stomach acid also helps to break the food down into smaller pieces, so it can be absorbed in your intestines.
The stomach makes the acid by separating neutral salts into an acid and a base. The acid is hydrochloric acid, and the base is bicarbonate. The bicarbonate stays in the stomach lining, to protect the cells there from the acid. The acid can then digest the food, without digesting the stomach itself.
Sometimes the acid in the stomach gets into the esophagus, the tube leading into the stomach. If you have eaten too much, and lie down, this is more likely to happen. The esophagus does not have the protective bicarbonate that the stomach lining has, so the acid hurts the delicate esophageal walls. We call this heartburn, or acid reflux. You have to sit up, or stand up, until your stomach empties enough to lie down and not spill acid into the esophagus.
Another way an acid can be “not good” is if it is no longer as acidic as we want it to be. The acid in batteries is needed for them to work properly. If it is diluted, or neutralized, then the batteries won’t work.