As far as I've gathered (not official), this is the MEETING time onboard the ship, so that group will disembark together to go on the excursion.
Prior to disembarking, I would guess they would go over the excursion's schedule, get everyone's names, etc - you know, all the admin. stuff. That way the group is ready to go when they get off the ship.
Anyway, that's just what I've seen through some of the posts.
- Jim