The iMessage upgrades that Apple includes in iOS 10 are making their way into your Mac thanks to macOS Sierra.
You can send big emoji, send a tapback message, preview links in the conversation and even play a video right inside messages so you don't have to spent the time to open a separate app and find space for it on your screen.
With the update you also get access to over 100 new macOS Sierra emoji that you can send to the iPhone and iPad. If you use Messages to talk to other people on the iPhone you will want this update so that you can see the new emojis they send.