Reactions allow you to express your thanks and let your team know that you need help. They're easy, fun, and give your community a new and clutter-free way to engage with each other on Stack Overflow. For example, rather than ending up with a long list of comments stating "thank you," you can have a list of users who thanked the author conveniently displayed and keep the comments free for follow-up. Reactions can be added to both questions and answers.
The available reactions are:
Reacting to a post is easy! Just click on the smiley face icon under the usual voting controls to the left of any post and then click on the reactions you want to add. You can add any number of reactions to each post, even all of them.
As reactions are added by users, a count of the reactions will appear below the smiley face. Clicking on that count will display a list of users that reacted to the post and how they reacted.