I don't understand the "Send @username a message" setting

[incapaz (sjr04)]

I was exploring around my instance of Discourse, configuring it, and found this setting in /review/settings:

However I don’t really understand what it means, can someone clarify it for me? Been searching around here, but if @{username} is in your search query makes it search for posts made by the user with that username, even removing it yields no results.

[Qursch]

When you flag a post, that is one of the options.

I believe this can also alert moderators, which is why it is on the review list.