Hi Sharon,
I often wondered this myself until my dear friend and fellow Moms Panelist, Darcie, explained it all. On the Disney Wonder and Magic, there is a fish on the door. This is used by the cast members to leave tickets for excursions or other messages. It is your stateroom mailbox.
Guests decided to "extend" the mailbox and began creating "extenders" to hang on the door. These same guests also decided to connect with other guests who will be sailing on the ship at the same time prior to departure. It became a tradition to bring a gift to share with your new friends and some of the gifts were magnets.
If you would like more information, you can search Disney Cruise Fish Extenders and you can read all about this cool tradition. (Darcie's take is on there:))
PS I think the Dream and Fantasy will continue this fun tradition.
Wishing you a magical day!
Beth