From AndroidCentral.com….
Google is rolling out an update to the YouTube mobile app that’ll give you more control over the video quality (via Android Police). It’s a bit of a step-up from what users had before on phones, but it basically lets you decide how the YouTube app uses your mobile data. Previously you could choose either “auto” or just choose the precise resolution you wanted, think 480p, 720p, or 1080p.
Now, you have a more complex settings menu.
This makes you able to choose auto, higher picture quality, data saver, or advanced. It’s a bit of a change, but it does put more power in the hand of the users, albeit needlessly complicating what used to be a simple thing.