Slaves reduce the maintaince cost of units and buildings by 12 gold per hour per unit so if you have 100 slaves then they produce 1200 gold but they consume food, slaves can be aquired by conquering other races city and enslaving its citizens (click on any building and then click enslave, building is destroyed after that)

Note: after you start constructing any building cities race is automatically switched to yours so if you want to enslave population dont start building anything there untill you are done