Truman is claiming that if the United States does not help Greece and Turkey, they will eventually fall to communism.
This excerpt is from a speech given by Truman after World War II and is the basis of the Truman Doctrine. After World War II, the US was worried about the Soviet Union and the system of communism spreading all over Europe. If communism spread, it would increase the Soviet Union's power, making them the world's largest superpower.
The US did not want this to happen, as they feared that if one country fell to communism, nearby countries would fall as well. This is known as the domino theory. To prevent this from happening, the US ends up giving $400 million to Greece and Turkey in order to stabilize their economy.