There's no less than 10 jillion articles on the internet these days titled "What is the Fiscal Cliff?". Unfortunately, almost none actually answer the question.
Here is the simple answer.
Federal Reserve Chairman Ben Bernanke came up with the term “fiscal cliff” while urging
Congress to avoid a steep dropoff (as in "cliff")for the economy at the end of 2012 thanks to tax increases and automatic budget cuts.
If you want to find out more about it, please read any of the aforementioned articles.