"The subjective sweetness of a wine is determined by the interaction of several factors, including the amount of sugar in the wine, but also the relative levels of alcohol, acids, and tannins. Sugars and alcohol enhance a wine's sweetness; acids (sourness) and bitter tannins counteract it." - Wikipedia
Residual sugar refers to how much sugar is left in the wine after it is fermented. This amount tells you how sweet the wine is.
Commonly measured in grams or sugar per litre of wine, residual sugar is found in almost all wine, even dry wine because some types of sugar cannot be fermented. A wine that contains over 45 g/L is considered a sweet wine. Other aspects that influence the sweetness of wine include the acidity and alcohol levels, tannins, and whether it is a sparkling wine. If the level of acidity is high, a sweet wine may taste drier and if the level of alcohol is high, a dry wine may taste sweeter.
