When you receive a message, you can react with one of five options. You can use reactions to acknowledge the message, answer a question, or provide feedback.
How to react to a message
When you're logged in on web or the app, react to any message by clicking the reactions icon below the message. These will only appear for the most recent message. Tapping previous messages will bring up the menu. Here's what the icon looks like:
You can react to a message with more than one option, but you can only use each reaction once. Tapping or clicking a reaction for a second time will undo the action.
If you receive messages by text and have internet access on your phone, you can react through the link that appears at the bottom of the message. This link will appear for all messages that contain attachments and all messages shorter than 95 characters (so that there's enough space to include the link to react).
How to see who's reacted
On Web hover over each reaction to see who reacted.
On iOS or Android you can long press the reactions to see the list.