The status "Seen" and a green tick appears under a message once it has been seen by its recipient. In the same way, other participants also members will also see whether you have read messages they have sent you. If a message has been sent but not yet read, the status "Sent" and a grey tick will appear below it.
In group chats with several participants, this read confirmation will only appear once everyone has seen the message.
You can deactivate this feature in the settings: Go to message settings
Please note: If you deactivate the read confirmation you also won't be able to see if people have read your messages.