Trocken is the German word for dry. It is used on German wine labels to indicate those which are dry rather than off-dry (halbtrocken ), sweeter (lieblich ) or sweet (süss ). Technically trocken wines are not completely devoid of residual sugar, but have at most a few grams per liter.
While historically most German wines were probably dry, for most of the twentieth century the style has typically been one of sweeter low alcohol wines. Only since about 1990 has dry, trocken wine become popular, partly as it tends to go better with food. Most German wine that is exported is still of the sweeter styles.