An ISO image is an archive file of a CD/DVD.
Sometimes you may need to burn an Acronis ISO image to a CD. This may be an ISO image that you created using an Acronis product, or a custom ISO image that you received from Acronis Customer Central. You can also download an ISO image for your registered product from Acronis website: see Downloading Acronis Bootable Media.
However the CD burning software preinstalled on your computer may only allow to burn regular files.
You can use a specialized freeware ISO burning program:
(!) Note that it is absolutely normal, if the Windows Explorer does not show any files on the CD after burning.