Answer:
Ranitidine is an anti acid, used for people with gastroparesis or stomach ulcer, also used for acid reflux patience.
If by antacid you mean the tablets people chew to neutralize too much stomach acid, like Tums or Rolaids, it is not that kind of antacid. Instead of neutralizing the acid that's already there, it prevents your stomach from making so much acid. Here is a description of how it works, from a medical website:
Ranitidine acts in the stomach to decrease the production of stomach acid. It works by blocking histamine H2 receptors that are found on the cells in the stomach lining. A natural body chemical called histamine normally binds to these receptors, causing the cells to produce stomach acid. By blocking the H2 receptors, ranitidine prevents histamine from binding to them. This stops the cells from producing stomach acid.