This is most often a
warning on a
UPS that
indicates that there is something
wrong with the
electrical ground. (In the
US, that would be the round pin on the bottom of the
plug) The most common
problem would be that the ground in your wall
outlet isn't actually connected to
ground and has a non-
zero electrical
charge to it. This could also be caused by a
short-circuit or a faulty
Uninterruptable Power Supply.
There are also electrical test devices that can detect this problem.