A joint session occurs during the mediation when both parties are in the room. Although it can be difficult for people to be in the room together, a joint session is an important part of the mediation. For example, sometimes parties need to have the experience of telling their story to the other side. This can only be achieved in a joint session.
It may be that the parties really need to share their perspectives and discuss the dispute in detail. This can only happen in the joint session. The direct conversation is extremely important to achieving the promise of mediation.
Mediators are able to act as shuttle diplomats moving between different rooms, but most strongly prefer it if the mediation provides the parties with a means of communicating with each other. The mediator can communicate an offer to the other side, but it will have significantly more impact (for both sides) if communicated in a joint session.