You want to enter a road displaying this road sign. To what does the information "12 t" refer?

You are correct that the "12 t" on this road sign refers to the actual weight of the vehicle. In German traffic rules, this sign indicates a weight limit, meaning vehicles with an actual weight (the current weight including cargo, passengers, fuel, etc.) over 12 tons are not allowed to enter that road. It is important to understand that this refers to the vehicle's real-time or actual weight at the moment, not the gross vehicle weight rating (a maximum legal weight) or the unladen (empty) weight of the vehicle. This helps protect road infrastructure from damage caused by excessively heavy vehicles.