Tags can be set by the user to easily categorize messages in an easy understandable way. Several tags per message are possible. They can be accessed in direct chat aswell as in a separate task menu to have an overview over all set tags.
The tags are illustrated as different colored dots, arranged around the field of a text message.
According to Donald Norman's aspect of 'affordance' they give the user a visual clue to their function which suggests him to tap on one or multiple dots to tag the text message in order to categorize it.