Rennie contains magnesium carbonate and calcium carbonate. That's fairly standard antacid remedy, because it's pretty good at neutralising stomach acid, which is hydrochloric acid.
CaCO3 + 2HCl → CaCl2 + H2CO3
Nothing wrong with MgCl2 or CaCl2 (all of those ions are vital for the body's metabolic functions anyway). It's the benign little molecule produced in both reactions: H2CO3. That's carbonic acid. Now, anyone who has studied environmental geochemistry (and any A2 student daft enough to ask) knows that:
This is the carbonate buffer system, which maintains the pH of the oceans. It's also involved in the maintenance of our internal pH (though it seems to be more frequently called the bicarbonate buffer system in physiology). It's an equilibrium, shifting to the right when the pH increases (in other words, the concentration of H+ ions decreases). Which means that, when the pH is low (the conditions are acidic), the equilibrium will move to the left.
What's on the left hand side? Well that would be water, H2O. We knew about that from the Rennie advert. But this other molecule, CO2, is carbon dioxide. The gas produced when we belch and fart. This is simply the standard chemistry behind the action of antacids, and shouldn't be a surprise to anyone with some high school science in them. It's tickled me that the advert is so coy about it, but I suppose if we were told that indigestion remedies leave us trumping like a one-man oompah band none of us would run out and buy them.