Some messaging apps and social networks let users add reactions to messages, like a thumbs-up, heart, or other emojis. It’s a convenient and quick way to show your response without typing out a full message.
When a user reacts to your message, a note with the same emoji appears in the case on behalf of that user.

You can track the addition of a reaction using rules for updated cases. To do this, specify the conditions "Changes were made by — user" and "Note was added."

For more details about how reactions are shown and handled in rules, see the knowledge base article here.