But, CDOs go a step further. You may have more or less risk tolerance. So we chop it up. You can buy something that gets paid back if 70% of the loans are good, or only if over 70% or over 80%, or even over 90%. The less likely you are to get paid back, the more you make.
So the person who absolutely wants to get paid back gets a small interest rate. The person who only gets paid back if literally every loan pays off gets a much higher rate.
In 2008, what happened is all those people buying the "very safe" 75-85% failure rate sections also were getting wiped out.
https://www.investopedia.com/terms/c/cdo.asp
Or if you prefer the Big Short explanation:
You take a bunch of loans and bundle them together and securitize the bundle to sell to the public. Mortgage CDOs crashed the global economy in 2007-8.