During their "aging" process in the bottle wines gain a series of different and precious characters, mainly bouquet (smell) and taste (meaning the way your palate and tongue react to it). Well, the longer time in the bottle the less they need to "breathe". In this case breathe means not more than oxidation. Oxidation, as you probably know, is the the addition of oxygen to a compound with a loss of electrons. This process is generated differently (slowly) inside of the bottle, but it occurs, so the "Younger" the wine the more breathing it needs. For a very young wine, you can open the bottle many hours before your dining, for an "old" wine can be only a hour or maybe less. In fact if a very old wine breathes too long it will probably collapse (chemically-wise) losing all its properties. It's also important to know, that in some cases, breathing is not good enough. You also have to decant the wine, this means pouring the whole bottle to a jug or jar. The aggitation summed to the aeration (or breathing) will release previously closed aromas and "open" more flavors. Again "decanting" is more important in young wines. Finally, red wines are usually the ones that need breathing. Salud !! (Cheers !!)