President Truman believed that poverty and suffering create conditions in which communism might spread because people who are poor are more likely to like the idea of communism than people who are well off.
Communism believes that people should not really own any serious private property. Communists think that there should not be rich people and poor people. Instead, communism promises that everyone will be equal. It promises that the government will own all of the factories and workplaces and that the government will give everyone enough money to live on. The government will make sure that everyone has enough and no one has too much.
If you were poor and suffering, wouldn’t that sound like it might be a good idea? The communists would promise you that the government would take care of you and make sure that you would not be poor anymore. If you believed them, you would be very likely to support communism. Conversely, if you were well off, you probably would not like this idea at all. Communists would be likely to take away some of what you have and give it to the people who are poor. You would not like that and you would not feel like it would help you if everyone were equal.
For these reasons, poor people are more likely to approve of communism. That means that poverty and suffering make it more likely that communism will spread.