Antacids (or indigestion tablets) are used to help stop indigestion. You may get indigestion if you have eaten too much or if you have eaten too fast. If you have indigestion, the liquid in your stomach will become more acidic.
The reaction that is happening is called neutralisation. This is because the tablets are alkaline and the liquids in your stomach are acidic.
An example of an Antacid is Milk of Magnesia.