Each of the five members of the UN Permanent Security Council had veto power over any resolution, including the resolution repeatedly introduced by the Soviet Union to allow the revolutionary communist government of China to take its rightful place in China's seat at the Security Council table.
The US just threatened to veto each resolution, essentially permanently denying China their seat. The Soviets left in protest, saying a UN that dysfunctional did not deserve their participation. The timing was simply bad, as the Korean War began and the UN voted to intervene before the Soviet ambassador could return and veto the resolution.
The reason that the Soviet Union was boycotting the United Nations Security Council meetings at this time had to do with the issue of China.
In 1949, the Chinese Communists had won the civil war that they were fighting against the Kuomintang nationalists. The Kuomintang had then fled to the island of Taiwan, where they set up their own government.
At that point, both the mainland, communist Chinese and the Nationalists on Taiwan claimed to be the real government of China. The UN was recognizing the Nationalists at this point and so the USSR boycotted.