What is mean by negative DC voltage?

Dual Voltage Supply

With older Opp amps dual voltages potentials were needed 5 V+, 0V and 5 V- you need a dual voltage power supply that can supply a positive, zero and negative voltage potential -a simple method is to use a voltage divider of two resistors in serie between the positive and negative where the point between the resistors will be your 0V the negative will be the negative voltage potential say -5 V and the positive is your positive voltage potential of +5 V.

Answer

DC voltage potential 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 potential.

Comment

Both of the above answers are incorrect, because they are actually referring to potential, and not voltage! 'Voltage' is synonymous with potential difference, not potential. In each case, you should substitute 'potential' for 'voltage' -as shown.