Phantom Power   

A condensor microphone needs a polarizing voltage applied to one of the plates in its diaphram. This voltage known as phantom power can be supplied by the mixer and sent down the microphone cable to the microphone. The mixer will either have a switch that turns on phantom power to all the microphone inputs or an independent switch for each input. Phantom power voltage is typically 48 volts DC. 
Several types of microphones, especially condensor microphones,  have a very high impedance which require that the circuitry which operates them be very close to the diaphragm of the microphone.  The power necessary to run these circuits  may come from a small battery in the microphone or from another external device (such as the mixer or the recorder).  This is called phantom power and is a direct current of (usually) 48 volts which is sent on the same cable which connects the microphone to its preamp. 
Because audio is an alternating current (AC), the direct current (DC) of the phantom power can be kept separate from the signal by means of a transformer
It is important to check the precise requirements of any given microphone as the voltage requirements can range from 1.5 - 50 volts and the pins used may differ. 

Also is recommended that phantom power be turned off when using dynamic microphones and especially when using microphones with their own internal battery supplies.