"A stable is any building in which horses are kept; a military stable would also include training grounds, barracks for the troopers, blacksmiths, armories, and the like. A horse must be extensively trained if it is to be successfully used in battle. In their natural state, most horses quite sensibly seek to run away from dangerous situations - say, when surrounded by screaming men armed with sharp objects. A cavalrymen's horse, on the other hand, needs to be able to suppress this instinct and instead instantly carry out any order given it by its rider - no matter how suicidal. Such training takes time and skill - for both mount and rider. Stables provide such training: any civilization which plans to employ large numbers of horsemen would be well-advised to make the investment and build as many stables as possible."
"Stables provide your mounted units with additional experience."
In the city screen press the following icon to start Stable construction: