Glasnost (along with its counterpart Perestroika) was one of the reform efforts of the Soviet Union. It meant "openess" in Russian and was meant to be a loosing of the restrictions on freedom of speach in the USSR. Coined by Soviet General Secretary Mikhail Gorbachev around 1986, it was significant in that it was one precedent of the ultimate collapse of the Soviet Union. Depending on one's stance it could be argued to have played a major role it this collapse or a minor one, and all shades of grey in between. Nevertheless it remains an important part.
As noted in Wilkinson (552) this reform actually worked to hinder its counterpart "perestoika" (restructuring). As the administration opened itself to criticism, it is noted that this hindered a restructuring.