How did glasnost and perestroika contribute to the fall of the Soviet Union?

They began a liberalisation process which led to the fall of communism and the break up of the Soviet Union.
Glasnost and perestroika were policies of political and economic reform introduced by Gorbachev. They were attempts to liberalise the Soviet system through political and economic reform which initially were aimed at maintaining communist control.
However once begun the process could only take two courses. Either it would lead to genuine democratic change as the demands of the public for greater social economic and political freedom grew; or these demands would be crushed through force if the authorities felt their elitist position was threatened.
It was the former of the two options which prevailed. Within a very short period of time, the prevalent order in Eastern Europe established after World War 2 collapsed.
The Berlin Wall came down and Germany and Berlin were re-united. Czechoslovakia became the Czech Republic and Slovakia, and the Soviet Union collapsed.
However such momentous changes were not without problems. The emergence of Russian nationalism and the current political structure means a lack of many basic rights. The collapse of Yugoslavia led to bitter conflicts in which over 500,000 were killed. There have also been bloody wars in other ex-Soviet republics such as Chechnya and the emergence of Islamic fundamentalism.