A Soviet Republic was an administrative reigon within the Soviet Union
In its final decades of its existence, the Soviet Union consisted of 15 Soviet Socialist Republics (SSR). They are all independent countries now, only very loosely organized under the heading Commonwealth of Independent States.
The USSR was a highly centralised and authoritarian entity. Under the constitution adopted in 1936 and modified along the way until October 1977, the political foundation of the Soviet Union was formed by the Soviets (Councils) of People's Deputies. These existed at all levels of the administrative hierarchy, with the Soviet Union as a whole under the nominal control of the Supreme Soviet of the USSR, located in Moscow. The role of the Soviets in the individual republics and other territories was primarily to put into effect the decisions made by the Supreme Soviet of the USSR.
Russia is by far the largest in area, it spans both Europe and Asia. Of the other 14, Kazakhstan is by far the largest. Russia is also by far the most populated, Ukraine, Uzbekistan and Kazakhstan are next, in that order.
See also: Socialist republic