Treasure Isle: Davy Jones' Locker doubles as storage and decor
To use the Locker, just click on any item and select the 'Move' option. Drag the item to the Locker, and click your mouse to place it into storage. But there are limits though. First of all, it can only hold 5 items. Getting 5 extra spaces, costs 20 Island Cash. You can also enlist the help of neighbors who respond to your wall posts or Facebook requests. Also, the Locker isn't allowed to hold everything. For example, relics, building items, eggs, and gem trees aren't allowed in there.
For more official details on the Locker, check out this Zynga thread.