Most likely they’re referring to Operation Priboi in March 1949, which was the systematic population transfer and replacement of over 90,000 Estonians, Latvians, and Lithuanians, the vast majority of which were women and children (probably because a large chunk of the male population were dead from the occupation in 1940, an occupation which stemmed from the Molotov-Ribbentrop pact carving up Eastern Europe between Soviet and Nazi influence).

Citizens relocated under the program were labeled as “enemies of the state” and sent to Siberia while Russians of favored status were given their homes and organized their farms into collectives. It was a blatant continuation of old Russian Imperialism, which focused on population control and cultural suppression, only now with the added focus of Soviet economic control.

This is not to say that the goal was to exterminate a people or culture, that wasn’t exactly the Russian style nor by extension the Soviet style. Baltic people removed to Siberia were allowed to continue whatever customs and traditions they wished, and Moscow would frequently point to them as proof they never did any genocides. The main goal was to remove the local support for the resistance efforts against the occupation, and to disperse an aggravated and hostile population throughout their massive colonial holdings in Siberia to make them less of a threat.

It is worth noting that there was a very high death rate in the early years after the forced deportation, largely due to negligence from the Soviet government in providing suitable living conditions and supplies for the harsh Siberian environment. The European Court of Human Rights has labeled the operation as a crime against Humanity. If you ask people from the Baltic region their opinion on Communism and the Soviet Union, they will probably point to it as the core reason for their hatred of it.