YouTube's Subscription Feed traditionally begins with a "Today" banner, presenting users with a reverse chronological list of every video that has been posted by the YouTubers they subscribe to, going back to "Yesterday," "This Week," "This Month," etc. For those in the experiment, this order is replaced with what Team YouTube calls a "personalized order," which appears to use a viewer's watch history and other factors to recommend videos from their subscriptions that the company thinks the user will want to watch.
Just to clarify. We are currently experimenting with how to show content in the subs feed. We find that some viewers are able to more easily find the videos they want to watch when we order the subs feed in a personalized order vs always showing most recent video first.— Team YouTube (@TeamYouTube) May 23, 2018
Using algorithms to surface content has long been popular among social media networks. Facebook's News Feed has done this for years, and Instagram followed in March 2016 stating that as it's grown its users "often don’t see the posts [they] might care about the most," although the company has made slight tweaks to the algorithms since then. For its part, Twitter as a whole still shows tweets from new to old, but it does choose to surface non-chronological content with features like "In case you missed it," displaying followers' liked tweets among your own, ads, and more.