Definition - What does Browning mean?
Browning is used to describe a wine's color. When a wine begins to turn to shades of brown, this typically means that the wine is past its ideal maturity and is beginning to fade. Slight tinges of brown does not mean the wine has gone bad and they can still be enjoyed.
WineFrog explains Browning
All wines red, white and even rosé wines will eventually turn brown. This browning process happens due to oxygenation over time.
White wines, even if they start off as pale in color, will eventually turn darker shades of yellow, to amber and then brown. Red wines tend to get lighter and lose their pigment of red, turning a brickish red to terracotta. They will also eventually turn brown. If a wine has turned completely brown, this is a sign the wine has passed its age and is usually not enjoyable. The exceptions of wines which are still good if they are brown or have browned are certain fortified wines; Tawny Port, Madeira, Marsala and some styles of Sherry. Sweet, late harvest, dessert wines are often still very enjoyable if they show browning also.
Some wine experts can tell the estimated age of a wine by its color.