Basically, it's someone from Pittsburgh:
"Yinzer is a term used to describe people who identify themselves with the city of Pittsburgh and its traditions. The word is derived from the Pittsburghese second-person plural vernacular “yinz”.
Historically, it was used to identify the typical blue-collar people from the Pittsburgh region who often spoke with a heavy Pittsburghese accent 1. Over time, the term has been used by many Pittsburgh residents to self-identify, even if they don’t speak with a thick accent.
The concept and use of the word gained popularity in the 21st century as the area’s population loss slowed, and the city became a hub for revitalization 1. As the city gained note as a desirable place to live, more outsiders have moved or returned to the Pittsburgh metropolitan area.
The term has taken on a connotation to identify someone who, for better or worse, is either a lifelong Pittsburgher, or says a phrase or commits an act that could be identified as something a stereotypical Pittsburgher might do."