"Rescued Items" are anything that got left in the invisible Tempory Items folder. Applications use this folder for well, temporary files. When the application shuts down properly, it is supposed to delete any and all of it's temporary files.
Files remaining in this folder are (in theory) the result of an application or system crash. They get moved to the Trash on bootup in case you can salvage any of your work from them. In practice, not all applications properly perform the cleanup every time. For example, iCab often leaves download temp files.
Unless you know that you have lost a document in a crash, just ignore that Rescued Items folder. It will go away the next time you empty the trash.