in the HELP menu in iTunes I found this, note the last sentence:
Play a Genius playlist or Genius Mix
iTunes can create playlists—called “Genius playlists”—from songs in your library that go great together.
In addition to creating playlists based on particular songs, iTunes can turn your library into a set of commercial-free radio stations playing your favorite music from your library. This is called a “Genius Mix.”
To create Genius playlists and Genius Mixes, iTunes uses anonymous information about your library and other iTunes libraries.
To create Genius playlists and use Genius Mixes, you must turn on Genius, be connected to the Internet, and have an Apple ID. Genius is a free service.
Note: If you subscribe to iTunes Match, Genius is turned off.