Public decks are re-usable by default. This means that other users who view your deck will have the option to make a copy of your content for editing and remixing in their own account. This option is only avaialble to decks with Public or Restricted privacy settings. When this box is not checked, viewers of your deck will not be able to copy it to their account for editing on their own.
No matter which 'reusable' setting you choose, as the author of a deck you will always be able to open, edit, or duplicate your own decks provided you have not reached the deck limit prescribed by your subscription-level.