Websites like YouTube and Netflix allow for optimized playback to your Chromecast. This is the preferred experience as optimized playback allows for video playback in 1080p with 5.1 surround sound versus the use of the casting the tab which has a limit of 720p video playback and stereo sound. The message is to inform you that there is preferred casting option present within the web page.
Learn more about the difference between optimized playback and casting a tab.
Need more help? Ask other Chromecast users and top contributors in the Chromecast help forum.