A2/AD stands for anti-access/aerial denial. There is a recently published article from The National Interest that laid out a potential scenario concerning such an effort in the Baltic Sea. It is only 4-pages and makes for a good read: Entering the Bear’s Lair: Russia’s A2/D2 Bubble in the Baltic Sea
There are a number of NATO members on the Baltic Sea: Denmark, Germany, Poland, Lithuania, Latvia and Estonia. Non-NATO members are Sweden, Finland and Russia.
Kaliningrad is part of Russia. It is the old German city of Konigsberg and surrounding former Prussian territory. It was given to the Soviet Union at the end of World War II and they attached it to Russian SFSR (which became the independent country of Russia in 1991). The Kaliningrad Oblast had a population of 941,873 in 2010. It is named after the Chairman of the Presidium of the Supreme Soviet of the USSR, Mikhail Kalinin (1975-1946), one of the original Bolsheviks.
This keys off of the previous post by Shawn about the “Third Offset Strategy,” of which A2/AD is a part of.