Hard Disk Error??????????? SOS

[monde]

When I turn on pc, get this error message on screen.

SMART Failure Predicted on Hard Disk 0: WDC AC36400L
Warning: Immediately back up your data and replace your hard disk drive.
A failure may be imminent.
Press F1 to continue.

What does that mean?
SOS required.Thanks

[MR COMPUTER]

S.M.A.R.T. is a little hd checking utility,turned on in your bios.The few times i have seen this message,the hd was dying.I would back up my data first.Then run scandisk .You may need a hard drive.