While there are other types on the market, (so I don't know what is in the specific A320) the Draeger Oxycrew is a very popular model that I'm familiar with, I would think others are similar.
The system has a canister of potassium superoxide (KO2). When your breath passes through the canister the CO2 and water vapour in your breath react with the KO2 to release oxygen. This is effectively "regenerating" the air into something you can breath.
There is some tech stuff at
this link