AC/DC refers to alternating current and direct current electrical systems. When an electrical appliance is labelled AC/DC, this means that it will work equally well on either an alternating current or a direct current supply.
In the past, the electricity supplies to homes and businesses might use direct current or alternating current, hence the need for AC/DC appliances. Thomas Edison favored the direct current system, while George Westinghouse favored the alternating current system. Each system had its advantages and disadvantages but the AC system became the dominant one and is now almost universal.