This command has several uses. In its most common use it renames a file. To rename your file junk so the new name is junk1, do

   mv junk junk1
After you do this, list your directory with ls to verify the name change, and list the contents of the file with cat or more to verify that the contents are nonetheless unchanged.

A second very common use is to move files to a different directory. To move a file junk to an existing directory called recycling, do

   mv junk recycling