An eruv (plural: eruvin) is a virtually invisible unbroken demarcation of an area.
In the Northeastern US, many eruvin specifically use materials that are indistinguishable from the materials commonly found on utility poles. Often, the choice of material is mandated by the utility company.
An eruv is required because Jewish law prohibits the carrying or pushing of objects from a "private domain", such as a home, to the "public domain" on the Sabbath and Yom Kippur.