Twitter blog:

We’re exploring ways to make the Twitter experience next level—and Twitter Blue is just the beginning. This opt-in monthly subscription offers exclusive access to premium features that let you customize your Twitter experience.

Twitter Blue is currently launched in Canada and Australia as an iOS in-app-purchase, with Android coming “in the near future”.

As to features:

Right now, Twitter Blue subscribers get access to Bookmark Folders, Reader Mode, Custom App Icons, Themes, and an Undo Tweet option. They’ll also get early access to whatever we’re testing in Labs where we explore more premium features and learn what subscribers want to include in their experience!

Follow the headline link for details.