Broadly, an SCA household is a group of people who are friends who gather together for mutual support or interest.
Typically, a household will hang around together at day events and camp together at camping events. There can be as few as two members or as many as several hundred. They might call themselves a household, a ship, or a clan.
Households don't have any official status in the SCA. You don't have to get anyone's permission to form one. Households come and go. They are a natural and recognized part of SCA life.