Ask HN: Big Data Software, What's in it?

Hey guys I keep hearing a lot about "Big Data". I have no idea what it is other than people doing sentiment analysis on Twitter, and Business Analytics Software.

Is this a niche that a freelancer could take up? If so, what are the prerequisite knowledge or credentials you need to earn?

Also what is this big data software about anyways? It seems to me just a fancy charting/graphing software. Is there an open source version of it?

Curious to learn more.