The phrase "to rein in" means to restrain, control or bring something under control. It can also mean to curb or reduce the power or influence of someone or something. The term comes from horse riding where a rider uses reins to guide and control their horse by pulling on the reins to shorten the distance between the bit in the horses mouth and the bridle, causing the horse to slow down or stop. In everyday language, "to rein in" is often used figuratively when someone needs to restrain themselves from doing something excessive or uncontrolled such as spending too much money, talking too much or behaving recklessly.