Fall Grün was a plan made by Germany to invade Czechoslovakia (a country in Europe) in 1938. They made this plan because they wanted to take control of the country and its resources. This plan was kept secret, but some people found out about it and tried to warn the Czechoslovakian government.
On March 15, 1939, Germany invaded Czechoslovakia and took control of the country. This was the result of the Fall Grün plan. It was a very bad thing for the people of Czechoslovakia because they lost their freedom and had to follow Germany's rules. The invasion also led to the start of World War II, which caused a lot of destruction and suffering all over the world.
So, in simple terms, Fall Grün was a plan made by Germany to take over Czechoslovakia and it caused a lot of harm to the people of that country and led to the start of World War II.