Google is testing a new feature in its Messages app that allows you to react to messages using emoji. First seen by Reddit users in a thread that was picked up by Android PoliceThe feature reportedly allows you to react to a message by long-pressing it. Then you can select from emoji, including thumbs up, thumbs down, anger, and crying with laughter. The feature appears to work only through RCS and is not available in SMS-only conversations.
Messages is not the first chat app to get emoji reactions, which are also available in Facebook Messenger, Twitter direct messages and Apple's iMessage. Similar to iMessage, if your phone does not currently have the feature (because it is not part of the test, for example), you will receive a separate message to tell you if someone has used an emoji to reply to one of your messages, instead of seeing the messages appear emoji next to the message itself. It seems like a workaround similar to the one iMessage uses for its green bubble users.
For now, Google appears to be testing the feature with a small number of users. However, it is unclear whether running the latest beta version of the app is a prerequisite for viewing emoji reactions or if it is a service side flag set by Google.