What does the U stand for in USSR?

"USSR" is the English form, and it was the abbreviation for "Union of Soviet Socialist Republics". In Russian it was "Soyuz Sovetskikh Sotsialisticheskikh Respublik", but the Cyrillic character for the sound of the letter S looks like the Latin character C, and the Cyrillic character representing the sound of the letter R looks like the Latin character P, so to someone used to English the Russian abbreviation would have looked like "CCCP".

U stands for Union. Where as USSR is the abbreviation for "Union of Soviet Socialist Republics"