It means that Mac cannot find the device that it is stored on. This kinda goes back to how Mac is based on UNIX. While booting, it has to find the disk and partition it is stored on so that it can start loading things like kexts and programs from there.
This is a difficult problem to solve; there are so many causes. We need to know some more about your system.
First thing you can do, though, is be sure your BIOS is set for ACHI.
what if i can't change my bios or hard drive to ACHI?