Homes are a collection of zones and rooms.
Every room or zone must be contained within a specific home.
HomeKit is able to handle multiple homes.
If you are setting up accessories in an office, then your office building would be a home.
A home needs to have at least one room. If you don’t define a specific room, HomeKit uses its so-called standard room.
Zones are optional, but useful as organizational tools.
One home is designated as the primary home and is the default home for Siri commands that don’t specify a home.