It means what it says.
OS X needs about 10 gigs of hard drive space for normal OS operations - things like virtual memory, temporary files and so on. Without this space your Mac will slow down as the OS hunts for space on the disk, files will be fragmented, also slowing things down, apps will crash and the risk of data corruption - that is damage to your files, photos, music - increases exponentially. Your first priority is to make more space on that HD. Nothing else can be done until you do. Purchase an external HD and move your Photos and Music to it. Both iPhoto and iTunes can run perfectly well with the Library on an external disk.