If someone suggests you let your glass of wine sit for a few minutes to open up, it means that the wine needs to interact with air in order to reveal all of its flavors and aromas. Allowing time to breathe is often necessary for full-bodied, cellar-aged reds, which tend to come out of the bottle somewhat "closed" until they've had a chance to aerate. Opening up is actually a fascinating process, since you can witness a wine change from so-so to smashing within minutes.