An Eruv is a technical boundary that allows Jews to carry in public areas on Shabbat. It is a tradition which has blossomed from a basic Torah principle into a highly complicated legal matter.
The concept of an Eruv goes back to the principle of Shabbat rest. Under Jewish law on Shabbat, it is forbidden to carry anything -- regardless of its weight, size or purpose -- from a “private” domain into a “public” one or vice versa. 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 after synagogue and take things with them — including their babies! They want to get together to learn, to socialise and to be a community.
Given the design of many communities in the past, many neighbourhoods or even cities were walled for security. As such, the whole area was regarded as “private,” and carrying was allowed.
Now we live in large sprawling metropolises, it’s not clear where the edge of our community is. The answer is a technical enclosure which surrounds both private and hitherto public domains and thus creates a large private area in which carrying is permitted on Shabbat. This is known as an Eruv.