Letting Wines Breathe
Letting a wine breathe is the process of exposing a newly opened bottle of wine to oxygen. When wine is exposed to oxygen it accelerates the aging process which in red wines helps to 'open up' the wine revealing aromas and flavours previously masked by high levels of tannins.
As this process oxidizes the wine care must be taken as to the length of time a wine needs to breathe or whether it should breathe at all. Young red wines can benefit from an hour or so of breathing due to their higher levels of tannins. Older wines will often be better without any breathing as even a short time can be detrimental to a wine pushing it "over the hill".