Glasnost is a Russian word that loosely translates as "openness," or "transparency." This was the policy pursued by Gorbachev when he acted as General Secretary of the Communist Party and therefore de facto head of the Soviet Union. To most Soviet citizens, the institutions of government and their workings were shrouded in secrecy. In this one-party dictatorship, judicial proceedings, for example, were often conducted behind closed doors, meaning that many people could never find out what had happened to family members charged with crimes against the state.
Gorbachev wanted to change all that, and under the policy of glasnost, the previously mysterious inner workings of Soviet government slowly but surely became more transparent to the very people that the system was supposed to serve. In addition, glasnost encouraged people to speak out about abuses in the system and to be more vocal in criticizing certain aspects of the regime, even criticizing Soviet leaders themselves.
As one can imagine, this had a huge impact on the lives of Soviet citizens. Previously, they'd not been allowed to criticize the system of government, at least not openly. Anyone who did so could find themselves in serious trouble with the authorities. But now, under glasnost, Soviet citizens could feel more confident in coming forward and expressing concerns about the way that the country was being run.
However, the greatest impact of glasnost came in the Eastern Bloc countries, which were firmly under Soviet control. Although glasnost only officially applied to the Soviet Union, citizens of Eastern Europe saw the new spirit of openness unleashed by Gorbachev as an opportunity to criticize their own governments. As with the Soviet Union, all the governments of the Eastern Bloc countries were communist dictatorships, which maintained an iron grip on power through repression and the denial of basic human rights. Communist states depended for their very existence on a lack of openness and transparency. Party officials worried that if people were allowed to criticize the government, then they'd surely demand more personal and economic freedoms—the kind of freedoms that ruling communist parties were unwilling or unable to give.
And that's precisely what happened in Eastern Europe. Glasnost acted as an inspiration to millions who wanted a radical alternative to the grinding poverty and repression of life under communism. Though it took another few years before the communist regimes of Eastern Europe started to collapse, glasnost had undoubtedly provided the catalyst for their eventual dissolution.
Ironically, it was the very nature of communist rule which ultimately proved its undoing. Communist states had always been rigidly top-down in their approach to governance, and glasnost was no exception. It came right from the very top of the political power structure and filtered its way down to the people. The legitimation of greater openness at the very highest level unwittingly gave the green light to those further down the pecking order to be more bold in their demands for change. This is not what Gorbachev originally intended, but it was too late to put the genie back in the bottle. The spirit of glasnost, and all it represented, could not be contained, and it was just a matter of time before it led to a general demand by the oppressed masses of Eastern Europe to get rid of the communist system altogether.