A diuretic is anything — food, drink, or medicine — that increases the flow of urine. In other words, it makes you pee.
Usually, going "number one" is no big deal, but sometimes people have trouble going. That's where diuretics come in: they make you urinate more. A common reason to take diuretics would be a problem with your kidneys. Taking a diuretic can be very important in getting excess water and also toxins out of your body. Coffee and alcohol aren't usually recommended by doctors, but they're also diuretics since they tend to make you urinate.