Find my iPhone is a special type of service, that blocks your iPhone, iPad, iPod or Apple Watch when it gets lost or stolen. The blockade is activated automaticly when you turn the Find my iPhone function on.
Are you buying a used iPhone or other Apple device? Check if FMI (Find my iPhone) blockade is turned on. Without previous owner's login detils you won't be able to use the device. All Apple products that have an IMEI number are supported.
You can check iPhone iCloud by Serial Number also.