Single-family zoning means that only one house can be built on a piece of land. It's kind of like having a rule that says you can only have one toy per kid.
Let's say there's a big piece of land where no houses have been built yet. In places that have single-family zoning rules, the people who own the land can only build one house there. This is different from places that don't have this rule, where people can build lots of houses on a piece of land.
The idea behind having this rule is that it's supposed to make neighborhoods quieter and more peaceful, because there are fewer people and houses around. It's kind of like having a library where everyone has to be quiet, instead of a playground where everyone is running around and making noise.
But sometimes people don't like single-family zoning because it can make it harder for lower-income families to find a place to live. If there's only one house allowed on a big piece of land, that house might be really expensive to buy or rent. It's kind of like saying that only one kid can play with the toy building set, and that kid has to be really rich.
So, single-family zoning can be good for some things, like making neighborhoods quiet, but it can also make it hard for people to find a place to live if they don't have a lot of money.