Disk First Aid can't repair my disk. Please Help!

I have a Clamshell iBook with 300MHz G3 processor. It runs OS 9 very well.

Yesterday, when I was attempting to browse the internet using my Mac at home, it stopped to response. Thus I have to press down the power button for a moment to shut it down. After that, when I was trying to start it up again, a Disk First Aid dialog box showed up and I tried to repair my disk. Later, it told me that "ERROR: Disk First Aid found problems with your startup disk that it can't repair. Back up your disk now." Then my Mac just started up normally.

What does that mean? What can I do to fix that problem? Please Help!

Have a nice day!