It means that the BIOS cannot read anything (or anything meaningful) from the Master Boot Record of the first hard drive. It may be that the disc has failed, or that it is physically fine but the contents of the MBR are corrupt.
Have you just installed an OS, or has this error come out of no-where?
You should try a rescue live Distro such as you get on the LXF disk sometimes, or Smart Boot Manager (SBM) which came with (eg) the LXF172 DVD. They should tell you what OS's (if any) are on your various partitions. Depending on results, you should/might then be able to rebuild your MBR.
Unsolved mysteries of the Universe, No 13 :-
How many remakes of Anna Karenina does the World need?