"Browning" is a term used to describe the color of a mature—and possibly fading—wine. It's not necessarily a bad sign, just an indication that the wine has reached its peak and is heading to that great vineyard in the sky (though it still can be quite enjoyable). You can see the color best at the edge of your glass, where it will look sort of dirty, ruddy, and brickish. Browning is, however, a bad sign for young red wines and all white wines, as it's an indication that the wine may have suffered overexposure to oxygen, faulty winemaking, or premature aging due to the fungus Botrytis cinerea.