A PPI is a "proton pump inhibitor". It is the newest class of drug for treating gastro esophageal reflux disease (GERD or heartburn). It works by inhibiting the cells in the stomach from pumping Hydrogen ions into the stomach. This keeps the acid level in the stomach down and will typically prevent heartburn.
They are not good for immediate relief of heartburn because after you feel the heartburn, the acid is already too high. That is why PPI's like Prilosec OTC says that you must use if for 14 days and that it is not intended to treat occasional heartburn.