So, West Berlin was under the control of the USA, UK, and France, while the Soviets controlled the eastern part. The Soviets blocked railroad ways, canals, and roads to West Berlin so that those in that part of Germany didn't have access to humanitarian supplies. The only way to get supplies in was by airplane, which was supposed to be a dangerous task, however, the Russians didn't try to stop the planes from coming in in fear of retaliation from the US and her allies. This incident occurred during the Cold War Era, a time in which the West (USA) and her allies feared that Communism would spread.
Hopefully this helped and good luck.