CO2 purging is a technique where you remove the oxygen in a bottle/can/keg in order to prevent oxidation of your beer.
CO2-purging can be done in multiple ways, usually through using pressurized CO2.
Counter-pressure taps (like the Tapcooler) and Beerguns lets you purge bottles by connecting a CO2 container.
Kegs can be purged directly through connecting a COS container and letting out the oxygen that will accumulate on the top. Kegs can also be “naturally purged” through connecting a pressurized fermenter to it during fermentation and letting out the excess pressure with a spunding valve.