The unread messages email is a summary email that gathers your unread chat messages into one place so you do not miss anything while away from the app.
It is a catch-up email sent to help you stay on top of chat when you have not been in the app. It brings your unread messages together in a single summary.
The email greets you by first name and summarizes how many unread messages you have across how many conversations. It lists each conversation with a "{count} new" badge and a preview of who said what, and ends with an Open your messages button that takes you straight to your chats.