power spectral density plots
The plot is not of power.  The units of the $y$ axis are dB/Hz. The dB part means that a decibel scale is used, which means it is a scale relative to a reference value.

All that negative values on a decibel scale mean is that the actual power is less than the reference power (because the ratio is less than one).

The higher the decibel value, the more powerful the signal.

Your signal has a major peak at about 4Hz, and lesser peaks at 28Hz and 39Hz.  There might be something happening at about 12Hz, but it's hard to say without more information (or data).