Many Twitter users have noticed that Twitter is now inserting tweets into their timelines that seemingly don’t belong. This is not an accident. Twitter has updated its help document, “What’s a Twitter timeline?”

In addition to the basic, essential definition of a Twitter timeline—“all Tweets from those you have chosen to follow on Twitter”—plus retweets and ads, there’s a new section:

Additionally, when we identify a Tweet, an account to follow, or other content that’s popular or relevant, we may add it to your timeline. This means you will sometimes see Tweets from accounts you don’t follow. We select each Tweet using a variety of signals, including how popular it is and how people in your network are interacting with it. Our goal is to make your home timeline even more relevant and interesting.

In most cases, these seem to be tweets favorited, but not retweeted, by people you follow. That change has concerned some users: Will everything they favorite—even semi-private tweets—be shown to their followers? It seems the answer is no; Twitter is only looking to highlight popular or relevant content.

Facebook’s newsfeed only shows you a fraction of posts from your friends and reorders the posts based on how relevant they are to you. Twitter has traditionally shown you all tweets from people you follow in strict reverse chronological order. The new text makes clear that while that dynamic isn’t changing now, Twitter will also be adding tweets you might like.

Still, it’s a fundamental change in how Twitter works. And it’s probably a sign of things to come: Twitter must tweak things to drive growth, it has a new product lead running the show, and has promised many new “experiments” to follow.