Users who use the unmentioning feature will have their username untagged from original tweets and replies. This will not prevent others from continuing the conversation, but will stop notifications for the thread and will prevent anyone in it from tagging you again.
The company has been working on this feature for more than a year. Twitter began testing unmentioning in April among a small group of users. At that time, they explained that it would help get rid of unwanted attention, which in some cases could lead to harassment or bullying on social media.
To unmention yourself from the conversations you no longer want to be a part of, you need to select the tweet you're tagged in and tap the three dots at the top-right corner of the screen. You should then choose the Leave this conversation option.