A boarding house is a place where people who are not related to each other live together in one big house. Just like when you go to a sleepover at your friend's house, you share a big house with other people, but it's not your house.
These people who live in a boarding house usually rent a room in the house from the person who owns the boarding house, and they pay money called rent for staying there. Each person has their own private bedroom where they sleep, and they usually share other things like the kitchen, living room, and bathrooms.
The owner of the boarding house is also responsible for making sure the people who live in the boarding house are safe and follow the rules. They might have rules like when to be quiet, when to eat dinner, and who is allowed to come and visit the house.
People who live in a boarding house usually stay there for a while, like for a few months, or even a few years. It's kind of like living in an apartment but with more people around.