Okay kiddo,
After the second big war, many nations saw that the economy was kind of a mess. A guy named John Maynard Keynes had an idea that if the government spent money, it could help make the economy grow. This worked pretty well for a while, and lots of people liked his ideas. This way of thinking about the economy was called "Keynesianism."
But as time went on, some people began to notice some problems with Keynesianism. One big problem was that government spending could lead to inflation, where the prices of things go up really quickly. Another problem was that some people thought that government spending might not always be the best way to help the economy grow.
So, over time, people started to look for different ways to run the economy. They started to pay more attention to the free market, where businesses compete with each other and people can buy and sell things without any interference from the government. They also started to look at other economic theories, like monetarism and supply-side economics.
So, in a nutshell, the post-war displacement of Keynesianism was when people started to question and move away from Keynesian economics, which had been popular for a while after the war, and started to look for other ways to run the economy.