A caucus is a private meeting between one side and the mediator. Usually, the mediator will keep whatever is said during the caucus confidential unless you give permission for them to share the information with the other side. Some mediators take the opposite approach and will only keep information confidential if you tell them to. Make sure you confirm with the mediator what their practice is before you share information during the caucus.
A caucus is great opportunity for you and the mediator. The mediator has the chance to get a deeper understanding of your interests and positions. They may make suggestions or help you to understand some of the information shared in the joint session. You may have information you felt uncomfortable sharing with the other side. A mediator can listen and may help you to find a way to share the information if that would be constructive.
If you have ideas as to how to settle the dispute but are too nervous to share them during the joint session a mediator can help you to consider what would be reasonable and also prepare how to make the offer to the other side.