Reply to invisible post

Hello,
what happened if I get a reply to the post “Hello world” which is the default post that’s always there at the start.
If someone replies to it but the post is not visible on the website what happened?
How is this possible?
Could it be a hacker?
If so what is the best pluggin for security against hacker?

This question might be quite basic as I am not very experienced.
I am happy about any advice and help though!!
Thanks a lot everyone!!!!