It essentially means using quantity instruments for expanding the balance sheet of the central bank. It can take different forms: quantitative easing (QE) through direct long-term asset purchase by the central bank, credit easing (CE) by the central bank directly intervening in a particular segment of the credit market.
Both QE and CE can be accompanied by dilution of collateral standards by the central bank expanding the list of collateral beyond its traditional preference for low risk sovereign bonds. Unconventional monetary policy is invariably accompanied by forward guidance regarding the future path of monetary policy to work on the expectations channel.