So, imagine you and your friend both have houses, and you want to live right next to each other. But there's not a lot of room, so you build your houses connected to each other like they're holding hands. That's what a back-to-back house is!
In a back-to-back house, there are two houses that share one wall. That wall is the back wall of one house and the front wall of the other house. It's called "back-to-back" because the houses are lined up with their backs against each other.
Sometimes, there are many back-to-back houses all lined up together in a long row. This is called a "terrace" or a "row of houses." They were often built a long time ago when there wasn't enough space for everyone to have their own big house with a yard.
Living in a back-to-back house means that you're really close to your neighbors. You might be able to hear them talking or walking around next door! But it can also be nice because you always have someone nearby to play with or talk to.