This folder holds the media files (images, sounds, videos, PDF documents...). These files can be uploaded through the backoffice and included into blog posts or other pages presented to the user.
This folder has different subfolders for different "media roots". Each of these roots can have different access permissions in the backoffice.
Depending on how you set up
/conf/_advanced.php, this folder may or may not need to be accessed through the web.