An open house can mean many things, but you're probably asking in regard to a home. Realtors will advertise an open house for a home that they're trying to sell. It is usually a few hours on a day of their choosing that folks can see the home without having a set time or appointment with the realtor. The aim is to get as many folks as possible to take a look and walk through of the house. The more people that look the more offers the realtors receive-hopefully.
since you posted this question in Jobs-Education: I presume a company would have an open house to offer the opportunity for unemployed people to come in and apply and find out if they even want to work there.