The banks kept betting on who will pay for their houses and who will stop paying and move out. Obviously, these applicants have a higher default rate than applicants with good jobs and good credit. No job, no income, no assets applicants (NINJA is their acronym/nickname) could get a house just because they wanted one. After this change, anyone could get a house. Before this change, you had to have a good job and good credit to get a loan to buy a house. So the banks changed the rules regarding who could buy a house. So, they wanted to sell as many houses as possible, because then they'd make as much money as possible, betting on them. The banks learned that when people don't buy as many houses, the bankers don't make as much money because they can't make new bets. The home-owners keeps making their house payments, the bankers keep winning the bets, the people betting on the bankers keep winning their bets, and everyone is happy. Other people see they're making a lot of money with this new 'betting on the housing market' thing so they come along and start making more bets on whether or not the bankers will win. The bankers figure out what percentage of people will stop paying. The banks see that most people want to pay. They gambled on whether or not people would pay for their homes or stop paying and move out. So they started another industry altogether - gambling.

I didn't feel like the other comments really explained the point of the movie. It's really hard to ELI5 this but I'll try.

Reddit the big short