Specifically, the Truman Doctrine had the following implications that worried the USSR:
1) The Truman Doctrine was a declaration of the US's commitment to help "free peoples who are resisting subjugation by armed minorities or outside pressures." This was a clear challenge to the Soviet Union, which was actively supporting communist revolutions around the world. The USSR feared that the Truman Doctrine would give the US a pretext to intervene in countries where the Soviet Union had interests.
2) The Truman Doctrine committed the US to a more active role in international affairs. This was a reversal of the US's traditional isolationist policy. The USSR feared that this would lead to a more aggressive US foreign policy, which could potentially lead to conflict with the Soviet Union.
3) The Truman Doctrine was popular in the United States. This meant that the US would be more likely to follow through on its commitments, even if it meant going to war. The USSR was concerned about the potential consequences of a war with the US, which was the world's only nuclear power.
In response to the Truman Doctrine, the USSR took several steps to strengthen its own position. These steps included:
1) Increasing its military spending.
2) Strengthening its alliances with Eastern European countries.
3) Supporting communist movements around the world.
4) Initiating the Berlin Blockade.
The Truman Doctrine and the Soviet Union's response to it marked the beginning of the Cold War, a period of intense rivalry between the US and the USSR that would last for over four decades.