It means the drive you're trying to boot from doesn't have a bootable OS or the OS is corrupted or the drive is dead or not connect. Basically, it means your system can't find an OS to boot from. Do you have a USB thumb drive in? Sometimes that will cause it to not boot either. Check the boot order in the BIOS. If you have multiple hard drives, check the order of those too.
The classic cause of this error message is that you left a floppy disk in the floppy disk drive. Nowadays it means that you left a bootable flash drive in the USB socket. The worst case cause is that your hard drive has failed.