Once you have created a document, you need to save it to the hard disk.  While you are working, the text and other items you may have entered is stored in the computer's RAM.  RAM is a a storage area that operates very quickly, but all data in RAM is lost when the computer is turned off.  You need to save the file, that is, write it to your hard disk, if you want to keep it.  Also, when you save a file the first time, you give it a name so that you can open it later.