They are sessions that are run simultaneously (e. g. Wednesday 12 pm, at the time of the weekly PASS hour for most groups) and primarily focus on a single course unit.
They group you up with other students that also opt to attend the module specific session and hopefully cover much more content that is relevant to you. If you can think of a module in which you're particularly behind or you think you're going to need some time discussing some concepts from one of your course units with your peers to be able to understand them, then these sessions would be a great opportunity to catch up on that specific module.