Answer:
Banks bundled mortgages together and then sold them on the market as a financial asset. However, the risk level of these securitized assets was often much higher than the purchaser thought.
Step-by-step explanation:
Securitization is a financial term that refers to the practice of bundling up various types of debt (for example, mortgages, auto loans, credit card debt, etc) into one single package, that is sold to a third party under a new financial structure (new terms of payment of both capital and interest).
The problem with this type of practice is that sometimes it groups together varios types of debt with different levels of risk that the third party does not assimilate or understand well.
Securization was one of the leading causes of the financial crisis of 2008-2009, as the question explains.