It was the Northern half of East Prussia, which was given to Russia after WW2.

The long history is that in the middle ages, Prussia was forcibly converted to Christianity by a Germanic military order. During the Protestant Reformation this territory secularized and became the Duchy of Prussia, which after about 350 years ended up defeating France in a war and leading the unification of Germany.

When Germany lost WW1, the Western parts of Prussia that were mostly populated by Polish speakers were given to the new state of Poland. The mostly German East Prussia remained part of Germany, but was not connected by land with the rest of Germany.

When Germany lost WW2, the Allies were much more confortable displacing German populations, and divided the remaining East Prussia into two pieces, with the Northern half becoming part of the USSR, and the Southern part becoming part of Poland. The Germans were generally expelled and Polish and Russian settlers replaced them. The USSR chose to make this area part of the Russian constituent country of the USSR, and when the USSR dissolved, it remained part of Russia.