Red envelope icon meaning in notifications menu?

[ondrej]

This is an official warning. It can only be issued by staff (admins and moderators)

You can read more about this in the Discourse moderator guide

Discourse Moderation Guide

So you’re a moderator on a Discourse forum - congratulations! As a forum moderator, it’s your role to help oversee the forum, ensuring that users and content are in line with your TOS, FAQ, and other published rules/guidelines. Discourse comes with numerous tools to help you accomplish this. Let’s take a look at some common scenarios and see how each can be handled with Discourse. Welcoming New Users Part of keeping a community friendly and helping it grow involves being an example to other…

As explained in the guide its a great way to try and correct user behaviour without going through the bother of silencing or suspending users.