Map found on the site of Poale Zedeck, a modern Orthodox synagogue in Squirrel Hill.
According to Wikipedia:
An eruv is a ritual enclosure that some communities construct in their neighborhoods as a way to permit Jewish residents or visitors to transfer objects from one domain type to another, such as carrying an object from indoors (a private domain) to a public street (a public domain) on Shabbat, which they would otherwise consider prohibited by Jewish law (Halakha). An eruv accomplishes this by integrating a number of private and public properties into one larger private domain.
The eruv allows these religious Jews to, among other things, carry keys, tissues, medicines, or babies with them, and use strollers and canes. The presence or absence of an eruv thus especially affects the lives of people with limited mobility and those responsible for taking care of babies and young children.