"Kernel panic" is, of course, just a situation where the kernel can't continue.
The phrase "not syncing" means that the panic occurred when the system was not frantically trying to flush unsaved data to disk (before finishing the panic). That's a Good Thing.
"Unable to mount root filesystem" usually means that the 'root=' parameter specified in the kernel command-line (in the boot loader, LILO or Grub) is missing or incorrect. If it is correct, then it may also mean that the correct filesystem driver isn't loaded: it can be compiled into the kernel or it can be loaded by the so-called "initrd" stage.
Messages which precede this one usually give sufficient clues as to the exact cause of the problem.