I have seen this sign mounted on the backs of some cars. But I could not find any information about it.
The upper end of the black part is spiky.
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Given that very similar sign is employed in Russia, I would theorise it means that this car has little spikes built in its wheels' tyres, facing outwards. Spikes allow car to come to halt much faster in snowy weather, which may be of surprise for drivers of non-equipped cars.
Spikes also supposedly damage roads' asphalt layer.
Please see a page describing this sign (in Russian).
The message descibes the season range when such tyres are allowed (1 Nov - 1 Apr), as well as features the sign itself.
I can confirm that in Lithuania these are are used in the same way as alamar described.
See point 5 here: https://www.keliueismotaisykles.info/transporto-priemoniu-skiriamieji-ir-informaciniai-zenklai-transporto-priemonese.html (in Lithuanian). This is the official site for traffic regulations in Lithuania. It states the needed size for the sign, as well as that any car with spikes on the tires is required to have this sign on the back of the car.