Google had said that autoplaying content with sound would be blocked by default in January's Chrome 64 update, but it was delayed. Instead, users received the ability to mute sound on a site-by-site basis---something that was supposed to arrive in Chrome 63.
Chrome 66 will still allow videos without sound or that are muted by default to play automatically. Unmuted autoplay will be allowed under certain conditions: when the window has been previously interacted by the user during that browsing session; if the site has been added to your Android device's home screen, and for desktop sites that a user frequently watches videos on. These sites are ranked by Chrome's Media Engagement Index (MEI), which measures an "individual's propensity to consume media on a site."
MEI is determined by a ratio of visits to significant media playback events per origin. There are certain conditions: a user must have played media for at least seven seconds, meaning any accidental plays that get stopped quickly aren't counted; the videos must have sound and not be muted, and the clips must be larger than 200x140 pixels, thereby discounting any small preview videos. You can check out your MEI at the chrome://media-engagement internal page.
When it comes to security features, Chrome 66 has removed trust for Symantec certificates due to the company's failure to adopt industry standards. The latest release also alerts users when third-party software injects code that results in browser crashes, along with a password management feature that allows users to save their credentials to a .csv file.
Chrome 66 also comes with Site Isolation---another mitigation technique for the Spectre vulnerability---along with numerous fixes for several other security issues, the details of which you can see here.