Dangerous goods are items that pose a hazard in flight: explosives, flammable liquids or gases, toxic or radioactive substances. Carrying dangerous goods is prohibited in the open category — it is one of the conditions that pushes an operation into the certified category.
An important nuance: the lithium batteries powering the drone itself are not "carried dangerous goods" in this sense. The ban is about what the drone transports as cargo, not about its own power system.