Phantom Power or +48V is a method of sending an electrical current through a microphone cable. It is most widely used as a power source for condenser microphones.
Condenser microphone capsules require preamplification in very close proximity. This requires a power source so that the signal can be amplified before traveling very far. In addition, true condenser microphones (as opposed to electret) require a voltage for polarizing the microphone’s transducer element. Phantom power provides a voltage for both these purposes.
Dynamic microphones do not require phantom power. Plugging a dynamic microphone into a preamp on which phantom power is enabled will neither affect the sound or damage the microphone. Ribbon microphones should never be plugged into a preamp with phantom power engaged.