Condenser microphones require power to their amplifiers to work properly. This is achieved by running a DC voltage directly down the microphone cable. The most common voltage for phantom power is 48 volts DC, but depending on the microphone, the operating voltage could be anywhere from 1 to 50 vDC.
Phantom power works by sending the same polarity down both the positive and negative lines of a microphone cable. Dynamic microphones are safe from phantom power because with only positive voltage going to the microphone, there is no way for the power to cross the coil, as there is no voltage potential without a negative lead.
NOTE: Do NOT send phantom down an unbalanced cable to a condenser, with only on positive, the transformer will ground and overland, resulting in a microphone shaped paper weight.
Always be aware of the input you are connecting the microphone to. Make sure the phantom is OFF when connecting dynamic, ribbon and condenser microphones.