Aeration is forcing air and wine to mix together for the purpose of bringing out the flavor of the wine. It also increases the aroma. Usually, the unwanted compound will evaporate quicker than the flavorful and aromatic compounds. Aeration can decrease sulfites. Sulfites are added to wine to prevent oxidation. However, sulfites can have a burnt smell. Another compound that gets reduced by aeration is ethanol. Ethanol can create a strong smell of alcohol, which you don’t want in your wine.
Some of the aeration technique is swirling your wine in the glass. After time, wines that are aerated will oxidize and flavors and aromas will surface. The denser the wine is, the longer is has to be aerated. As far as the older fine wines go, don’t aerate these for long as you can miss their aroma.