negative dc means voltage is at negative potential and
vice-versa for positive dc.

Negative & Positive polority plays major role in case of 
Electronics Devices.We may aware of simple electronics 
components Diode,Transistor,etc are made of PN junctions.
For proper conductions & baising,Blocking Polority of the 
circuits is very important

Negative & positive polarity are two different potential 
with repect to earth and the current flow's in different 
direction.

DC voltage has either -ve OR +ve polarity. in case,
you connect +ve polarity to ground and -ve polarity to
source, at that time it produces negative DC voltage.
      A negative DC supply usually is used along with an
positive supply in audio electronics in order to produce an
AC output (both positive and negative voltage excursions
with respect to the zero volt level) which represents the AC
nature of the typical audio input signal. The only other use
for a negative supply voltage that I can think of is mostly
a thing of the past but old fashioned ram chips required a
+5, +12, and -12v supplies.