It seems to be a cover all term for revealing the identity of someone who wants to remain anonymous.
So, for instance, Venice Allan was banned from Twitter, but re-registered under a different name, and Julie Bindle outed her on Twitter. Earning her another ban. I think that could constitute being doxxed.
Posey Parker has had her children's school publicised online. And photographs of her family, which were on a private Facebook page. That's another example.
A poster on here inadvertently posted something that contained her name, and a man on Twitter got hold of it and tweeted it to expose her.
There is also the possibility of online places being hacked in order to find out the identity of people.
The word dox, I believe comes from documents. Documents used to find out someone's identity, or details about their life that they would rather remain private.
It's usually used to intimidate. Talking about details that are already in the public domain doesn't usually mean doxed.
One of the reasons it's quite worrying is because a lot of women are speaking out about sexism/misogyny/transactivism and the way to stop them is to expose who they are online and encourage harassment.
The man I mentioned upthread has posted a photograph of the interior of a church. Pretending he's going to visit it. It's the church used by the partner (a priest) of an outspoken feminist. It's meant to intimidate and frighten.
I'm sure other people will think of more examples.