Centuries ago, sailors hewed to the idea that, on a ship, you used one hand for yourself and one hand for the ship. A “hand” gradually came to refer not just to the hands of the sailors doing the work, but to the sailors themselves. “Shout,” someone might call out, “and we will send another hand” – that is, another sailor.
When someone shouts “All hands on deck!”, it means everyone is needed, and there are crucial communications from the captain and other leaders to the crew. But the all-hands also lets the crew, or staff, ask questions of the leadership. Modern business uses the all-hands meeting in much the same way: for efficient, critical communications between leaders and team members.
The all-hands is arguably the second-most important meeting you can have, after your leadership meeting.