Before you start the work on CSS do this: copy the folder which contains your Web pages and images. It is always a good idea to keep older copies of a site in case you mess it up. Maybe rename the new folder musichtmlonly and keep it as a backup. Make sure that from now on you are editing the files in the original folder (do not ever change the backed up copies).
Cascading Style Sheets is a slightly weird way of saying "Let's put the styling for the page in a separate place". You will learn one of those places by pressing the right button above now.