The Marshall Plan was an initiative launched in 1947 in which the United States gave a whole ton of money (about $12 billion back then, which is like $96 billion in today's money) and various kinds of other aid (like food) to countries of western Europe. The idea behind the plan was to make sure that those countries were able to rebound economically from World War II. This was important to the US because we did not want those countries to become poor because that might tempt them towards becoming communist.
The aid was also offerred to countries in Eastern Europe, but the USSR forced them to turn it down. I assume that we offerred it to them mainly for show because we surely knew the Soviets would never let them take aid from us.