What is the use of antacid?

Indications
Antacids are taken by mouth to relieve heartburn (the major symptom of gastroesophageal reflux disease), or acid indigestion. Treatment with antacids alone is symptomatic and only justified for minor symptoms. Peptic ulcers may require H2-receptor antagonists or proton pump inhibitors.
The usefulness of many combinations of antacids is not clear, although the combination of magnesium and aluminum salts may prevent alteration of bowel habits.
Some associations containing bismuth salts (which may cause encephalopathy) or an anticholinergic substance are not advised.
Side effects

Aluminum hydroxide: may lead to the formation of insoluble aluminum-phosphate-complexes, with a risk for hypophosphatemia and osteomalacia. Although aluminum has a low gastrointestinal absorption, accumulation may occur in the presence of renal insufficiency. Aluminum-containing drugs may cause obstipation.
Magnesium hydroxide: has laxative properties. Magnesium may accumulate in patients with renal failure leading to hypermagnesemia, with cardiovascular and neurological complications.
Carbonate: regular high doses may cause alkalosis, which in turn may result in altered excretion of other drugs, and kidney stones. A chemical reaction between the carbonate and hydrochloric acid may produce carbon dioxide gas. This causes gastric distension which may not be well tolerated.
Calcium: compounds containing calcium may increase calcium output in the urine, which might be associated to renal stones. Calcium salts may cause obstipation.
Sodium: increased intake of sodium may be deleterious for arterial hypertension, heart failure and many renal diseases

It is a kind of drug.
When we fell stormache(胃痛），we will eat it to neutralize(中和）the acid in our storm.

Antacids are taken by mouth to relieve heartburn (the major symptom of gastroeso*****eal reflux disease), or acid indigestion. Treatment with antacids alone is symptomatic and only justified for minor symptoms. Peptic ulcers may require H2-receptor antagonists or proton pump inhibitors.