Say something online that other people don't agree with, and they may retaliate by making your personal information completely public, sometimes with catastrophic consequences. Such is the crux of doxing, which gained major notoriety thanks to the 2014 GamerGate controversy. To make a very long, tedious story short, GamerGate erupted when a woman released a video game that other serious gamers felt was detrimental in some way to the industry. To exact revenge, they posted the address, phone number and other personal tidbits about this woman (and ultimately others, too), which incited threats and harassment to such an extreme level that she actually had to leave her home [source: Hathaway]. The movement (if you can call it that) spawned "dox drops" on people who dared to criticize the doxing.
If you're wondering why releasing someone's name and address is such a big deal when most of our info is out there for all the world to see, the difference is that doxing makes such information readily available to the legions of people bent on doing harm to a particular person. It's literally at their keyboarding fingertips, making it terrifyingly easy for them to wreak havoc on someone's life and livelihood. In the words of sociologist and Feministing blogger, Katherine Cross, "To dox is to elevate certain data above others, highlighting it and thus painting a target on someone's back by making personal information – home and workplace addresses, phone numbers – easier to see." I'd insert a snarky comment here, but I'd like my personal info to stay that way, thank you very much.