The Prague Spring was a brief period of liberalization in Czechoslovakia during the Cold War. It occurred in 1968. Though it is called the Prague Spring, it lasted more than just through the spring. It is said to have started in January of 1968 and to have ended that August.
In 1956, Nikita Khrushchev, who was the leader of the Soviet Union, started a process that is known as “de-Stalinization.” What he wanted to do was to move the Soviet Union away from what it had been like under Joseph Stalin. He wanted society and the economy to be at least a little more open.
This attitude spread slowly through the Soviet bloc. By the mid- to late-1960s it had reached Czechoslovakia. The Czech communists, led at that point by Alexander Dubcek, wanted to open up their society and economy. The problem was that they wanted to open it more than the Soviets would allow. This led to a situation where the Soviets eventually intervened militarily to end the reforms.
This was an important event for at least two reasons. First, it delayed reforms in the communist bloc for decades. Second, it helped to deepen anti-Russian attitudes in Eastern Europe. These attitudes are partly responsible for the fact that countries in that region are typically trying to ally themselves with the US and the EU today.