A preliminary conference is usually held before the mediation between the mediator and the lawyers for the disputants, often with the disputants present also.
The purpose of the preliminary conference is to make sure everything is prepared for the mediation and that those who intend to participate in the mediation will have appropriate authority to enter into an agreement to resolve the dispute, without having to contact someone who will not be there.
If the disputants attend the preliminary conference, they have an opportunity to become acquainted with the mediator in an environment in which they are not called upon to make decisions about the substance of the dispute. This can help them feel more relaxed about the process of mediation (and the mediator) by the time the mediation actually starts.
For a list of topics usually considered at a typical preliminary conference, download the agenda PDF here.