Topping up simply means adding more liquid to a container to fill it to the top. In the case of wine, this is done to squeeze out the airspace, which would otherwise cause a wine to oxidize. Wine stored in a barrel will gradually evaporate because of the porous wood, so on occasion the barrels are topped up with wine— from the same vintage, varietal, and vineyard, of course—to prevent air from mixing with the wine (this explains the little holes on the top of the barrel). Other examples of topping up occur with Champagne, which needs to be topped up with dosage after disgorgement, and very old bottles of wine whose ullage has increased to the point that they need to be topped up and recorked.