The black dot on iMessage typically refers to a new incoming message that has not yet been read. When a message is received, the dot appears below the message icon in the main menu and remains there until the message has been opened. This feature provides users with a quick way to see which conversations have unread messages without having to navigate through each individual conversation. In iOS 16, the black dot will appear next to the profile thumbnail for any new incoming messages that have not yet been opened.