Lewis Ranieri is a man who helped invent a way for people to easily buy homes, even if they didn't have a lot of money. It's kind of like when you want a toy but you don't have enough money. Someone helps you borrow money so you can buy the toy and then you pay the money back over time.
Well, Lewis Ranieri helped create something called mortgage-backed securities. This is a fancy name for a bunch of home loans bundled together and sold to other people as an investment. So instead of one person lending money to someone for a home, lots of people could lend a little bit of money and together own a part of many homes.
The idea was that if people couldn't afford to buy a home outright, they could still invest in a piece of many homes and hopefully make some money over time. This made it easier for people to buy homes because banks had more money to lend out, and it also allowed people to invest in real estate without having to buy a whole house.
That's basically what Lewis Ranieri did - he helped make buying and owning homes easier for lots of people by creating a new way to invest in them.