He did things like gut the central planning institutions and slash the amounts of goods and materials the state was purchasing from factories, leaving them to trade amongst themselves and sell to whomever and at whatever price they wanted, eliminated price controls and subsidies, began selling off state assets to "coops" that were often just criminal organizations involved in the black market/"second economy" that the Soviet authorities had been turning a blind eye to since Krushchev, appointed anticommunist radicals to control the state media institutions, and abandoned socialist countries in the periphery as a policy of appeasement to the US.

His reasoning is a bit harder to pin down: he came to power as part of a wave of reform aimed at modernizing and improving the USSR's socialist economy, and initially followed the reform plan laid out by his predecessor, to decent success. The problem was his own ideas, however well intentioned, tended to backfire horribly, and so he began leaning on more outspoken reformists for concrete plans who were ultimately revealed to be anticommunist liberals.

Socialism Betrayed: Behind the Collapse of the Soviet Union goes into some more detail about what his reforms entailed, the conditions before and after them, the backgrounds of all the major figures, and the major events of the eighties. There are criticisms to be made of how much it places the blame on individuals rather than movements and material conditions, but it still has value as a reasonably thorough overview of what was going on in the USSR in the eighties.

He opened up the economy to global markets and created a new free press all at the same time in a shock therapy sort of way. Compare this to China who did the former in a very slow burn way, and never opened up their press.