Under Jewish law on Shabbat, it is forbidden to carry anything from a “private” domain into a “public” one or vice versa, or more than four cubits (approximately 6 feet) within a public domain. Private and public do not refer to ownership, rather to the nature of the area. An enclosed area is considered a private domain, whereas an open area is considered public for the purposes of these laws.
It became obvious even in ancient times, that on Shabbat, as on other days, there are certain things people wish to carry. People also want to get together with their friends and take things with them—including their babies. They want to get together to learn, to socialize and to be a community.
Enter the Eruv. An eruv is an enclosure which surrounds both private and hitherto public domains and thus creates a large private domain in which carrying is permitted on Shabbat. The eruv can be large enough to include entire neighborhoods with homes, apartments and synagogues, making it possible to carry on Shabbat, since one is never leaving one’s domain.
Technically, the eruv should be a wall. However, a wall can be a wall even if it has many doorways creating large open spaces. This means that a wall does not have to be solid and the eruv may be created by telephone poles, for example, which act as the vertical part of a door post in a wall, with the existing cables strung between the poles acting as the lintel of the doorframe. As such, the entire “wall” is actually a series of “doorways.” Added to that there may be existing natural boundaries and fences.