A bit of history is required to understand the immense connotations that this word carries today. Gorbachev came into power in 1986 and instituted a radical new strategy in the closed Communist state: the allowing of open political and social discussion, without any potential government backlash for doing so. Before this point, government censorship had been a central tenant of the Soviet government in order to maintain political stability.
Why did Gorbachev implement Glasnost?
This was done in an attempt to stop the rampant corruption within the Soviet government, by encouraging public oversight of government actions. The name for this policy was glasnost.
The intention of this policy was not to create overall freedom of speech, but that was the consequence: this dramatic reduction in the amount of censorship led to sudden increases in the flow of information in all aspects of Russian life. Suddenly, people were able to openly criticize the Soviet government, including many aspects that had previously been covered up such as rampant alcoholism, poor housing conditions, and many other broken promises of the Communists. Many of the more extreme Communist politicians were immediately removed from power for not supporting glasnost, while dissidents of the Soviets were released by the thousands from prison and were able to speak out openly. This increasing ferment of dissension eventually led to the collapse of the USSR; indeed, the failed coup that directly preceded this collapse was aimed at removing Gorbachev and his glasnost policy.