Series 6 is a level of certification for brokers granted by the Financial Industry Regulatory Authority (FINRA), allowing holders to conduct business in mutual funds and variable annuities.
FINRA is a non-governmental regulatory body overseeing securities firms in the U.S., in order to protect the public by ensuring brokers are qualified to do their jobs. FINRA administers more than 20 tests (tagged by a number preceded by the word "series") allowing brokers to engage in more and more complex market transactions. The serieses aren't numerically ordered, though, as Series 7 is generally the first test beginning brokers take.
The Series 6 exam is 100 multiple-choice questions, with at least 70 correct answers needed to pass. One-quarter of the questions deal with securities and tax regulations, and another quarter deal with product information for investment company securities and variable contracts. So those areas are where FINRA wants prospective license-holders to be most well versed. Other portions address evaluating customers and opening and servicing customer accounts.
Many organizations offer courses to prepare you for the Series 6 exam (as well as the other tests offered by FINRA) for a fee. It's like getting ready for the SAT or the bar exam.