The Marshall Plan could be understood as part of an American desire to stop the spread of communism.
After World War II, the US was worried about the growing size/influence of the Soviet Union and their communist system. Their main concern was that this system would spread to European countries and eventually the Soviet Union and their communist ideas would infiltrate the US.
To ensure that this did not happen, the US gave $13 billion worth of economic aid to European countries that were greatly affected by World War II. This included countries like Great Britain and France. This assistance would make sure that these countries did not fall under the control of the Soviet Union.