“Dismiss” just marks them as read. DEpending on your settings and where you’re looking at it, it will have different behavior. In your personal messages inbox, that just means that the notification will be marked as read, not removed from your notifications. Note that this doesn’t mean I have read the message.
If you’re in /new, it will dismiss them from view, because they’re not new anymore. They are still present. It’s just an unread notification action.
Example:
After dismissal:
The actual message has not been marked as read. See here:
“Dismiss” from the profile menu only affections notifications and their read state.