To file is to submit an application or a record to some official authority, like a court, police station, or city hall. You might file a complaint against your neighbor after he shovels the snow from his driveway into yours.
People file for divorce, file for bankruptcy, and file charges against criminals. You can also file a receipt or a record at home, by putting it in a file folder and then sliding the folder into a file cabinet. When you stand "single file," you're in a line or queue, and if the group moves a few steps, you can say it files forward.