Aeration is the process of letting wine "breathe," or exposing it to air before you drink it. This is often done with a lot of undue pomp and circumstance, the irony being that the vast majority of today's red wines benefit little from aerating. As a general rule, only youthful, high-quality white and red wines improve with aeration. Since older wines (20 plus years) have already softened their tannins through bottle age, they usually need very little aeration and should be enjoyed shortly after removing the cork before the quality of the wine diminishes due to over-aeration. By the way, just popping the cork and letting the bottle sit won't do the trick, because the wine won't get enough air to properly breathe. You need to pour the wine into a decanter or a large wineglass. See also decant.