The simple answer is that a moderator cleans up obscenity. The easiest way to describe it to her is to point her to two websites I happen to frequent.
is a website in which the moderators are constantly present. It is a site in which nobody would even think of posting anything even the least bit offensive, lest they be permabanned instantly.
is the exact opposite. DO NOT CLICK THAT LINK IF YOU ARE FAINT OF HEART. Ben Kenobi's words come to life on /b/. There truly is no greater hive of scum and villany. Pornographic pictures can be seen constanly. Hate speech flows like water. And the /b/-tards, as they call themselves, love it. This is an example of where there really are no moderators, and the few that are there don't do anything much at all.
So, if you need to tell her what a moderator does, point her to these two websites. She'll get the idea.