After World War II the country of Germany ended up dividing into two separate countries. East Germany became a communist country under the control of the Soviet Union. At the same time West Germany was a democratic country and allied with Britain, France, and the United States. The initial plan was that the country would eventually be reunited, but this didn't happen for a long time.
Berlin was the capital of Germany. Even though it was located in the eastern half of the country, the city was controlled by all four major powers; the Soviet Union, the United States, Britain, and France.
Finally, the Soviets and the East German leaders had had enough. On August 12th and 13th of 1961 they built a wall around Berlin to prevent people from leaving. At first the wall was just a barbed wire fence. Later it would be rebuilt with concrete blocks 12 feet high and four feet wide.