Ludwig von Mises and subsequent Austrians developed the theory that the business cycle is an unintended consequence of government intervention in the monetary and banking system. Specifically, the central bank (the Federal Reserve in the United States) pushes the interest rate down below its "natural" level by injecting new money into the banking system. This artificial stimulus sets in motion an unsustainable boom period of illusory prosperity.
During the subsequent (and inevitable) recession, resources are reallocated in light of the "malinvestments" made during the boom. Far from being "bad," the recession is part of the process of recovery, where entrepreneurs make the best of the untenable situation created during the boom. It is in this sense that Austrians say that recessions are a good thing. They are the recognition of the previous mistakes that entrepreneurs have made investing scarce resources, when they were misled by the distorted price signals reverberating from the Fed's interventions.
An Open Letter to Gary Becker re: Depressions - Robert P. Murphy - Mises Institute