An open house is an event where a place that's usually private opens its doors so visitors can look around freely, without needing an appointment or special invitation.

The most common type is a real estate open house. When someone wants to sell their home, they might hold an open house on a weekend afternoon. Instead of scheduling individual tours with each interested buyer, they open the house to everyone at once. Visitors can walk through the rooms, imagine living there, and ask the real estate agent questions. It's like a casual tour where multiple families might be looking around at the same time.

Schools also hold open houses, usually at the beginning of the year. Parents and students visit classrooms, meet teachers, and learn about what to expect. You might show your parents where you sit, where supplies are kept, and what projects are displayed on the walls. Teachers often explain their rules and goals for the year. It's a chance for families to see the school environment and feel connected to where their children spend their days.

The phrase suggests welcoming openness: doors that are usually closed are now open, and people who might otherwise need permission can simply walk in and explore.