A meta post(https://meta.stackexchange.com/q/257614/714660) says
Past experience had led us to believe that “small” and “unhealthy” would mean the same thing: we took it as a given that quiet sites would develop spam and obvious broken windows, while high quality sites would always grow big and graduate. But you proved us wrong! Five years later, we have lots of tiny sites which have been in public beta for months or years, each consistently producing excellent Q&A which helps people with real problems. Small, consistently active sites are great!
I googled "broken windows" and the first result is "The broken windows theory", which is obviously not the same meaning in that meta post.
I guess it is some kind of metaphor.
So, what does "broken windows" mean when talking about the quality of a site in stack exchange?