Peaceful co-existence is started by the soviet leader Khrushchev in the mid 50s, it was after Stalin's death and Khrushchev introduced the new policies of "De-Stalinization" and "peaceful co-existence". The new policy meant that the USSR was prepared to live together with USA in peace, instead of the constant threat of another world war.
2. Detent is a French word meaning relaxation, this name was given to the period of ten years between 1969 to 1979. During this decade both USA and USSR made effort to reduce the areas of tension between them.
3. The SALTs was made to reduce the arms force of the two great powers, Both USSR and USA agreed to cut down their Strategic weapons e.g. ICBMs, SLBMs, ABMs etc. SALT I was the first treaty about the cutting down of the missiles between USA and USSR. Five years later the SALT 1 was expire, so another treaty salt two was signed, it supposed to last until 1984, it contained the restriction of some new weapons, and the treaty was stopped when USSR invaded Afghanistan. The treaties were signed to show that they are friendly, but the number of war heads they had could still destroy the whole world.
4. The Soviet Union collapsed in eastern Europe in the late 80's. There are several reasons for it. In 1979 after a decade of the Detent, USSR invaded Afghanistan Americans were not happy about it, Reagan used it as a propaganda weapon to attack the soviet union by calling them the "evil empire". During this invasion USSR poured large sum of money into this military movement. The Russian economy wasn't good before the war and it just made it even worse. Therefore there weren't much support from the public inside the country. Also the war had made issues in Eastern Europe less important. President Reagan came in power in the 1980's and he decided to not to continue the policies of the Detent which made no true progress, and he believed that the only was way to end the cold war was to destroy USSR.