It could mean a lot of things:
1) You mashed really low, and used a very attenuative strain.
2) As mentioned, your hydrometer is out of calibration
3) You have an infected batch
It's not really rare to end up below 1.000 in certain cases like Saison using 3711, but unless you intended to end up below 1.000, it's likely one of the other reasons I listed. Mash temps might be the culprit if your thermometer isn't in calibration.