A microphone is an electric device that amplifies the sound of a voice or instrument. If you're going to stand up in front of a large crowd and want to be heard, you better use a microphone.
Microphones require electricity and amplifiers to work — what they do is take a sound and convert it into an electrical signal. That signal can then be amplified and sent to a speaker or recorded. Performers can sing softly into a microphone and still be heard clearly by an audience member at the very back of a huge concert hall. The popularity of radio and film inspired this meaning of microphone — originally, the word meant "ear trumpet for the hard-of-hearing."