An open interview is when during a certain block of time all candidates for a position who are able appear at the place of employment at the same time are on a first come-first serve biases interviewed one by one as opposed to individual candidate appointments.
Be prepared to answer questions about your employment and education history as well as explain why you are interested in the position and have a few copies of your resume.
(Source:
http://jobsearch.about.com/od/jobinterviewtypes/g/open-job-interview.htm)
Good luck, I've never done it this way before the very few times I've been invited for an interview.