The new Streaming feature, available for all Pro users, allows for users to begin streaming media prior to the completion of the download. In most cases, Streaming will begin once enough of the file has been downloaded.
For maximum efficiency, non-streamed torrents will normally download all parts of a file in random order. For example, you might get the pieces of the end of a song before the first part of the song downloads.
When a torrent is streamed, the download is forced to occur in sequence from front to back of the file. When enough of the file is downloaded, the media file begins to play, similar to a piece media that is buffered.