Note: The following article applies to SMF 1.* and above.
Many admin do not know what a recycle board is. This is an important feature as without this board any threads or posts that are deleted, either by a moderator or by a user, are gone forever and can never be restored unless a database backup has the post or thread that was deleted.
First step is to create a new board that either only admin [no groups checked] or moderators can view. To create a board go to admin panel -> boards and click the add board button.
After the board has been created go to the admin panel -> boards -> settings check the box enable recycling of deleted topics and in the next drop down select the board that was created above, dont forget when done to click the save button.
In SMF 2.* and above if any users that can view the recycle board have the permission to move topics they will find a restore topic link along with the normal moderation options which will put the thread back in the board it was originally. This feature is not available in any version prior to SMF 2.*.
It can be helpful to apply a permissions profile to this board to limit deleting topics from the board, but this is up to the admin to decide.