An eruv (Hebrew: עירוב, lit. "mixture") is a ritual enclousure around a Jewish community than enables observant Jews to carry certain objects outside their homes on Shabbat, an action that is normally forbidden due to the Biblical prohibitions surrounding "carrying" (hotzaah.) Eruvs are commonly built by combining a walls, fences and poles with wires on top to encircle a neighborhood, effectively turning these separate "domains" into one domain shared by all members of the community. Though the laws that go into building an eruv are extremely complex, there are eruvin in many Jewish communities.
Isn't this just a loophole?
In a way yes, but not in the way you might think. Carrying objects in most places is only forbidden on a Rabbinic level. So the same Rabbis who banned this type of carrying in the first place also created a way to allow it with certain limitations. Carrying that is forbidden on a Biblical level is always forbidden, and it is not possible to build an eruv as a "loophole" to get around this.
What do I do if the eruv is down?
Although Jewish communties go to great lengths to ensure that their eruv is up for every Shabbat, occasionally an eruv is "down" due to unforseen construction, accidents, or inclement weather. When an eruv is down, observant Jews are not allowed to carry objects such as books, keys or food. Pushing a stroller or wheelchair is similarly forbidden. If your eruv is down, you should contact your local Rabbi to figure out how to proceed.