Delete message Question

It’s under Navigation still isn’t intuitive to me….

Ahah! It’s not a countdown, but a count up. Now it says 18 min. I was trying to be funny, decided to delete it, and now can’t seem to do anything with it.

And what is a “hidden” reply? Who can see it? Just me or can who I replied to see it? I’m assuming everyone can’t, or else why call it hidden?