When the logarithmic magnitude scale was developed a few centuries ago (see the question "What is magnitude?"), its zero value was set at the earthquake threshold perceptible by seismographs at the time. However, measurement technology has improved in leaps and bounds tremendously since then, so today's seismographs can register earthquakes far below the old zero value.
The zero point on the magnitude scale has no special physical significance. It just means that a magnitude 0.0 earthquake is around twice as intense as a -0.2-magnitude earthquake and approximately half as intense as a magnitude 0.2 earthquake.
Very small earthquakes are called microquakes and fall way below the human perception threshold (see the questions "What is microseismicity?" and "When do earthquakes become perceptible?".
Earthquakes with magnitudes of -5.0 have been detected in deep boreholes of geothermal power plants.