This is the 9231st most frequent Russian word.
советский
"Soviet" (relating to the Soviet Union or its people, culture, or government).
Here, "советском" is used as an adjective to describe the time period associated with the Soviet Union.
Я прочитал интересную книгу о советском времени.
I read an interesting book about Soviet times.
The word "советские" is an adjective describing films made during the Soviet era.
Советские фильмы вызывают у меня ностальгию.
Soviet movies evoke nostalgia in me.
In this sentence, "советским" is an adjective modifying "гражданином," indicating the citizenship related to the Soviet Union.