Note: If your site is hosted by n390 this is not a concern for you. It's an issue only if you manage your own hosting or have direct access to the web site host server.
Never, ever let anyone tamper with them:
The Drupal core files, along with the database, are what makes the Drupal CMS application work. If you look in the web site root folder you can see all the files and such that make your site work. For the most part these are not your web site content as your site's content actually lives in the database.
Ideally a developer may upgrade the core files without affecting your site content, which is a huge benefit for productivity and maintenance.
Drupal Core Files: Files and folders from the top level directory of Drupal 5.7