What does 'Glasnost' mean?

Literally 'openness' - that is, a willingness to talk about things, even things that were embarrassing to the state. That was a big deal in the Soviet Union which had usually spent the previous 70 years trying to cover everything up. It even tried to cover up the German invasion in 1941. One of the first things it led to was a fairly open discussion about the mistakes that led to the Chernobyl disaster and the almost bigger mistakes made in trying to deal with it, which went a long way to discredit the Soviet government in the eyes of the people.