It's an easy way for a blogger to get feedback on posts. We'd all be driven by the responses we get from our readers, if we knew what those responses were.
By using Reactions, you can find out if your readers find your posts funny, interesting or cool. Those are the default choices when you sign up for Reactions, but you can change those options to suit your own needs. And you don't have to limit your Reactions to just three choices - you can have more if the need is there, or maybe you have a yes-or-no question you want answered. It's up to you, and you can change it as often as you wish.
With Reactions, your readers can give you feedback with just a click, allowing you to fine-tune your blog to attract more readers.
To get your own Reactions, just log into your blog's Dashboard, then go to Layout > Page Elements, and then to the Edit in the Blog Post element. Check the box next to Reactions, then add the specific Reactions you desire as a comma-separated list.
Click Save and you've added a really nifty tool that will give you the feedback you need to make your blog even better!