What are Reactions?
If you’ve used Slack or Thefacebook recently, you’ll have noticed a new way of interacting and providing feedback – emoji reactions. It works much the same way as a Like button, but provides a wider range of reactions, so readers can give more nuanced feedback, without needing to go to the effort of leaving a comment. This also allows for readers to provide the same level of interaction in situations where a “Like” is an inappropriate message to send, as Eric Meyer describes in his post about Inadvertent Algorithmic Cruelty.

What does it do?
The reactions plugin currently has the following features:

Allows for reactions to posts
REST API endpoints for storing and retrieving reactions
An exceedingly ugly emoji selector
What is still being worked on?
Pretty much everything!

In its current state, the plugin is mostly a proof of concept, in need of significant work improving edge cases, design and User Experience.

Your first step is to install the plugin, as well as the WP-API plugin (the WP-API plugin is currently a requirement to avoid code duplication, that will likely be re-evaluated based on when each plugin might be merged into Core).

Please report any issues on the Github repository, or drop in the #feature-reactions channel in Slack to ask questions or give feedback. It’s also where we have our weekly chats, on Wednesday, March 9, 2016, 3:00 PM PST. Thank you!

https://make.wordpress.org/core/2016/03/07/reactions/

Good point by @meiam.