Just a quick question about moderation; what is ''obscene''?

So I saw a question where an answer had been removed, because it was obscene. That’s the first time I see that as a reason for removing an answer. Although I can easily imagine, what exactly does ’‘obscene’’ entail? What does one have to post to get that? (not that I’m planning on getting it)
Please notice, I’m not asking this in order to whine about moderation, not for a second. I’m just really curious about obscene since I never saw it before, and I also thought it was kinda funny. XD