That means it can't mount the hard disk.
Most of the time this means the hard disk is damaged and you'll have to replace it, however, you can try a few things.
First you need to boot up from your Mac OS X install disk (hold C at startup to boot from the dvd drive). Then open disk utility, and try to verify and repair the disk. If it repairs it, you're good, if not, read on.
I always try DiskWarrior first, as it almost always works, and unless the drive is completely gone, it'll work. Since you're using a very old computer, I'm guessing it still has the original drive, and it's not at all surprising for an HDD of that age to just give like that. So you may want to just skip this, although DiskWarrior really is a steal at $99, it's saved me a bunch of times, and the bootable disk is always in my travel bag.
If none of the above work, you just plain need to buy a new hard drive, which you can get from iFixit right here. You can stick that in your laptop easily and be up and running in no time.
The main problem is your data will be gone, if you have a backup, you're fine, but if not, you may want to try data recovery. I have used DataRescue in the past, and it's worked great, it recovered a ton (gigabytes and gigabytes) of files from a fried hard drive I had a couple of years back. Again, it's not exactly the cheapest, but if your files are worth $99 to you, I'd go for it.
Those are pretty much your options, it's a pretty routine thing, if you have any questions, just comment below. If you found this answer helpful, please click "accept answer" so this question can be marked as resolved.