Renaissance person is someone who works at many careers during his/her lifetime, possibly at the same time. The "Renaissance" part is because a lot of people (usually men) were like this during that period, such as Leonardo da Vinci, who was an artist, scientist, inventor, engineer, architect, and a whole bunch of other things. I suppose that, as it was a time of change, there were more unfilled "niches" for people to fill, and some people just happened to fill many at once, not to mention how low the barriers to entry for those jobs probably were, for a couple reasons.
We may be entering another "Renaissance", with regards to the Internet. For example, I'm answering this question on
English.StackExchange, and I've answered several questions on
Security.StackExchange, and even
Physics.StackExchange (although I haven't gotten money for any of them).