Don't quote me on this for exactness, but if I recall the term right it means that everything the management of a company (or other organization) does is open to the employees or public to know about. So no secret meetings or anything is kept secret to anyone. Correct me if I am wrong.

An open door policy means, literally, that every manager's door is open to every employee. The purpose of an open door policy is to encourage open communication, feedback, and discussion about any matter of importance to an employee. When a company has this policy, employees are free to talk with any manager at any time. Companies adopt an open door policy to develop employee trust and to make certain that important information and feedback reach managers who can utilize the information to make changes in the workplace. An open door policy is normally a part of the employee handbook.

Thank you Dee and Rykuu's explanation!

To expand on previous comment: It could also mean that employees have access to management at all levels and are able to seek help or advice from their supervisors without fear of repercussions.