The rearrangement will show up only if you enable it. And it might not be that different from what you're already seeing under the "While you were away" section of your timeline.
Here are key things to know about what's changing:
The main change
Twitter has been showing tweets from people and organizations you follow in chronological order, with the newest tweets up top. With the change, posts that Twitter deems important will appear on top, even if they are older. What's underneath will still be presented chronologically.
Twitter defines it as "tweets you are likely to care about most." The service isn't revealing the specific formula, but says it will be based on factors such as accounts you interact with most and specific tweets you engage with. It will include only tweets from accounts you follow - not promoted tweets, which companies pay to insert into your feed.
These special, top tweets appear on top when you open Twitter's app or log on to its website. There's no label at the top or any marker separating these from normal, chronological tweets underneath. But you can tell by the time stamp that they aren't in real time. When you refresh your feed, you'll get the chronological tweets back, and these top tweets will move down your timeline and eventually drop off the page.
What about "while you were away"?
The "While you were away" feature was created for people who may have spent hours or days away from Twitter. The new top tweets section is targeted at people who aren't away from Twitter that long, but may follow hundreds or thousands of people and might not see tweets they may find relevant.
The changes will be offered to everyone on Twitter by the end of Wednesday. You'll need to go to your settings and choose "Show me the best Tweets first." Otherwise, nothing happens - for now. Over the next few weeks, Twitter will prompt you to either enable the feature or go to the settings to turn it off.
Will there be ads?
No brand can advertise with a promoted tweet in the top tweets section. But promoted tweets will continue to show up in the regular feed. If you follow a brand, a tweet could make it into the top section, just like any other tweet.
This isn't Twitter's first tweaking of its timeline. Twitter introduced "While you were away" last year and started allowing advertisers to pay for promoted tweets in 2010.
Other social networks
Facebook displays posts based on your connections and activities on Facebook, rather than chronologically, but you can customize the feed in various ways. Instagram, which is owned by Facebook, offers a chronological feed.