What is Oxydation?
Oxidation is caused by exposure to the oxygen in the air.
This occurs when wine is left open for several days or improperly stored. The wine may discolour, lose its aroma, and taste flat, but will probably still be drinkable.Nothing can be done once this has happened, but if you want to keep an open bottle oxidation can be avoided through the use of inert gas like Private Preserve.
Oxidation can also be used by design. For many wines a couple of hours air contact, in the bottle or a carafe, actually improves the wine. There are also types of wine where oxydation, either a little or a lot, is a key part, by design, of the wine profile. Jura "sous voile", Madeira, Amphora wines are some examples.