Rentier capitalism is when people or companies make money from owning things instead of working or producing things. Imagine you have a big toy box and you let other kids pay you some money to play with your toys. That's kind of like rentier capitalism. You didn't have to do much work, but you made money from other people using your things.
Now, imagine instead of a toy box, you owned a house. You could let someone live there and pay you rent every month. That's also rentier capitalism. You didn't have to work to make the money, but you profited from owning the house.
This kind of capitalism has become more common in recent years, where some people and companies make a lot of money by owning things like land, resources, or intellectual property. Instead of creating new things or solving problems, they make money by charging others for access to what they own.