On this road traffic sign, to what does the information “12 t” refer?

This traffic sign means that vehicles with an actual mass (current weight of the vehicle including load and people) over 12 metric tons are not allowed to enter. The restriction is based specifically on the actual mass while driving on the road, not on the maximum authorized mass stated in the registration papers and not just the weight of the load. This rule is designed to protect roads and bridges from being damaged by heavy vehicles. Always check your vehicle’s current total weight before entering such restricted zones.