With this release, Safari 9, it’s meant specifically for OS X Yosemite users. Apple says that the update is meant to address security concerns, as well as include improvements to compatibility and privacy. There are also new features baked into the new software, some of which are brought over from Apple’s release of OS X El Capitan, including the ability to mute audio from Safari tabs, as well as new viewing options for Safari’s Reader mode.
The update to Safari 9 should be available now for anyone running Yosemite. Apple is releasing OS X El Capitan today, on September 30.