In the Twitter blog post, they have been explaining this new feature in detail:
Before you Tweet, you’ll be able to choose who can reply with three options: everyone (standard Twitter, and the default setting), only people you follow, or only people you mention. Tweets with the latter two settings will be labeled and the reply icon will be grayed out so that it’s clear for people if they can’t reply. People who can’t reply will still be able to view, Retweet, Retweet with Comment, and like these Tweets.
In the blog post, Twitter says, “only a limited group of people globally on Twitter for iOS, Android, and twitter.com can Tweet with these settings, but everyone can see these conversations.”
“Being able to participate and understand what’s happening is key for useful public conversation. So, we’re exploring how we can improve these settings to give people more opportunities to weigh in while still giving people control over the conversations they start.”
“One thing we know for sure is that you’ll be creative with this update. Maybe you’ll host a debate on the benefits of pineapple on pizza (#TeamPineapple) with fellow pizza pals or invite a panel of distinguished guests for a fireside chat. You could even play a game of tic-tac-toe for people to follow along without messing up your moves. We’re excited to see what you do!”