What is the microphone problem in nikon d3100?

I have heard that there's no microphone in nikon d3100. What does it mean and in what way it will affect?

You shouldn't believe everything you hear. The Nikon D3100 has a built in monaural microphone.
http://www.nikonusa.com/....TechSpecs

You've heard? Have you read your owner's manual? If not, why not? If there's no microphone then you won't have audio on video, will you.