It can be address and phone number, but basically it's any private information that the person hasn't publicly released (this would also cover someone who keeps their public and private accounts separate,) - so, for example, giving details of where a person works, or who their family members are, would be considered doxxing if that person hasn't released those details on their public account. It can even be someone's real name if - like me, for example - they use a pseudonym. Doxxing creates the opportunity for harrassment (and possibly even physical danger) IRL, and it's generally a good idea to steer clear of it: people have a right to privacy.

Hope that's a more thorough explanation for you :)