In a direct message or a group, Seen appears under your newest message once the other person has opened the conversation since you sent it. In a group it names who has seen it. Team channels do not have read receipts.
Read receipts work both ways: you see when someone has read your messages only if you both have Read receipts on. It is on unless you turn it off in Account settings, under Privacy, on the website or in the app. In your conversation with Magpie support, staff can always see when you have read their replies.