To use the Reaction on Messenger, you have to tap a little button next to the messages you receive and choose one of the seven emojis listed there that is assigned to that particular message. This is similar to the way Apple implemented Tapback in iOS 10 Messages app.
A reliable source spoke to Facebook, who confirmed that the feature is on its way, and it’s been tested with a small group of Messenger users first, before rolling it out to the public. Though, the feature will be available for all Messenger users worldwide.
Let’s just wait for the official release of this feature. It’s going to be an interesting one, as there will be Like and Dislike emoji options too.