Alright, your cashier is just checking if your money is fake or not. If placed under the money detector, some parts of the banknote would normally glow and reveal interesting marks. The marks are called fluorescent prints and they "fluoresce" or glow when exposed to UV light.
The money detector holds a special bulb called the UV lamp or black light. It emits ultraviolet light which causes fluorescent dyes on the banknote to light up. Fluorescent marks usually portray the denomination of the banknote and is located off-center to the left of the portraits. The dye is also incorporated in some of the serial numbers and security fibers. This causes them to glow as well.