There are well circulated lists of dns names that are known to host various types of malware, and I am assuming this implements one or more of those lists. They have stuff like: botnet C&C domains, domains that have been hijacked to run exploits, domains that port scan the entire web just to run exploits at random, domains that mail out malware in spam, etc.

Or "Adult Content?"

Not sure, I wish it had more info on this. I tried some sites from big malware lists and 1.1.1.2 blocked none of them.

There are lists of such domains, like in Adblock