This feature is currently being referred to as LiteVideos, and it's specifically designed to reduce data consumption and improve performance while streaming videos.
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User "bugdroid" within the Chromium forums describes LiteVideos as a "data-savings optimization that simulates low bandwidth conditions by throttling media responses, to allow adaptive MSE video players to lower the resolution, and consume less data."
LiteVideos would be looking for videos that begin playing within the browser and automatically force them into a lower resolution. This may not seem particularly useful if you're not using mobile data or consuming the limited data attached to your plan, but it could be helpful for those with weak internet connections.
LiteVideos isn't currently available in any stable version of Google Chrome, but if you want to play around with it, you can download Chrome Canary. However, Canary can be quite unpredictable, so keep in mind that any bugs and issues found while using the feature will likely be patched when it's expected to officially go live later this year.
Once Canary is downloaded, copy and paste chrome://flags into your address bar and it should take you to an "Experiments" screen. Once you're here, type “Enable LiteVideos” and “Force LiteVideos decision” into the search bar and activate both.