In an official blogpost Google announced that it would be rolling out video previews when video results appear in the video carousel. “For videos, we have traditionally shown a static image thumbnail in search results. But as more information moves to video, we’re working on new ways to give you useful glimpses, helping you quickly find what you’re looking for across video, too.”
The auto playing video previews are currently available only on the Google app for Android and Chrome on Android. The feature will roll out widely in the coming days. To save data previews play only when connected to a Wi-Fi. To enable previews on mobile networks or to opt out of this feature, visit the settings menu within the Google app or settings for Android Chrome.