The Trash section of the Library is where you can move all of your unnecessary photos. It works just like a regular Recycle Bin.
When you delete a photo, Event, or Album from Phototheca, it is moved to the Trash first.
Items that are in the Trash can either be cleaned up and removed from the Trash or restored back to the Library.
To move an item into the Trash, select an item and hit the Delete key or click the Trash button on the toolbar.
To restore an item from the Trash, go to the Trash section, select one or several items, and click the Restore button on the toolbar.
To remove an item from the Trash and permanently delete it from Phototheca, select an item in the Trash and click the “Clean” button on the toolbar.
To delete all items that are currently in the Trash with a single step, use menu item File->Empty Trash.