Credit default swap
So many companies bought and subsequently sold CDSes that when Lehman collapsed, no one had enough money to pay the people they sold the CDSes to. They tried getting it from the companies they'd bought CDSes from, but they didn't have enough money either. Those companies tried collecting from the companies that owed them money, but they didn't have enough either, and so on. Since AIG had sold so many of these, people were afraid that AIG would just give up on trying to pay them all. If they had done that, there would be a domino effect where everybody would go out of business. Because so many companies would've gone out of business, the government decided to help AIG pay so the economy wouldn't collapse.