Simply put, these neutralize acid in the stomach. Antacids do not decrease acid production, rather work by temporarily neutralizing the acid that is in the stomach at the time they are taken.  They work quickly but do not have long lasting effects.

More Detail About The Actions of Antacids

Anything with a high concentration of loose hydrogen ions is an acid. Stomach juice contains a large number of hydrogen ions and is therefore, an acid. Antacids, which usually contain aluminum or magnesium, are weak bases.  Bases contain oxygen and hydrogen atoms that soak up loose hydrogen ions, converting them to water.  Because water is neutral and antacids convert the acid to water we say they neutralize the acid.  These drugs will affect only the acid that is in stomach at the time they are taken, as more acid is pumped into the stomach the effects begin to wear off.

Antacids containing magnesium can act as laxatives, whereas those containing aluminum may cause constipation.

Common antacids are: