Reactions allow users to React to a post from a choice of emojis, rather than only the Like heart.
A reaction can be created either by clicking the like icon OR by hovering and selecting a reaction from the reaction picker.
To remove a reaction, click the reaction again. (after 24 hours you can no longer do this)
On the left side of the post, there will be a list of reactions. Hovering on a reaction will show who used it. Clicking on any of the reactions will open up a list showing everyone who used any reaction on that post.
When should I use reactions?
You might want to congratulate someone but your post just isn’t long enough. Instead, use the 
reaction. Or you might want to applaud someone’s hard work. Use the 
reaction for that. Or if someone left a funny reply, just use the 
reaction. Ran out of daily likes? Use the 
reaction.