During barrel ageing some water and alcohol evaporate, thus leaving a gap in the barrel To prevent harmful contact between the wine and oxygen, the barrel is periodically topped up with wine of matching quality
or topping off - The addition of wine to fermatation vessels from a reserve supply to keep the container full The process reduces the possibility of oxidation Also used for adding wine at the bottling stage to completely fill a bottle
If you top something up, you make it full again when part of it has been used. We topped up the water tanks He topped her glass up after complaining she was a slow drinker. see also top-up