OK. So one of the things that many observant Jews don't do on Shabbat is "transfer between domains". It's one of the 39 traditional things you're forbidden to do. (Many of the modern prohibitions are interpretations or derivations of things on the list, but this is one of the literal ones.) What does it mean? Well, for all practical purposes, it means you can't carry things in public, or carry something from a public place to a private one or vice versa. And, again for all practical purposes, "public" means "outdoors". (Outdoor spaces that are legally private, such as a backyard, would have to be fenced in a certain way to count as private for our purposes, and indoor spaces that are kind of public, such as the hallways in an apartment building, can be considered private for this purpose via an easily-created loophole.)
So I and many other people who follow this rule have to take steps to structure our Shabbat to avoid problems. If I'm attending a potluck dinner on a Friday, for example, I'll make sure to bring my food before sunset, and I won't bring leftovers home. (My friends and I often run around right before sunset bringing food to each other's houses and quickly leaving, only to come back several hours later, relaxed and dressed fancier and empty-handed, ready to eat said food.) No reading in the park on a lazy Saturday afternoon, either. In order to avoid carrying my house key, I instead wear it on a string as a necklace, carefully put together such that the key is part of the necklace loop rather than hanging off of it, so it is truly part of the thing I'm wearing, and not something that my necklace is "carrying". Many people also consider pushing a baby carriage "carrying", which has helped to create some unfortunate gender implications of this rule over the years.
Well, this can be mighty inconvenient, so of course we've come up with a way around the problem. If one simply walls in an entire city neighborhood, one can consider it and all the houses in it "private", and therefore carry things all Shabbat long! (Well, there are other things you'd have to do, legal formulas to recite and all, and other restrictions you'd have to follow, such as not walling in areas as busy as Times Square, but I won't bore you with details. Anymore than I already am, that is.) So let's build a wall!
Except that that might disrupt traffic, bother the neighbors, and cost millions of dollars.
Well, actually, it doesn't. What if the wall in question has lots of gigantic doorways? With no doors in them? And the non-doorway parts of the wall are really thin? People might not even notice.
So that's what we do. We build the wall. In city settings, the "wall" is usually telephone poles and the "doorways" are the space between them. Any good doorway has to have a piece on top (because if it doesn't, it's just a gap in the wall), so the electrical wires serve that purpose. Since the top part has to be directly above (but not necessarily touching) the "wall" part, and since electrical wires tend not to go right over the tops of the poles, pieces of plywood about 3 feet high and 1 foot across are often nailed to the poles in such a way that the wires are going directly above them. If there are gaps in the wiring, one can instead string some fishing line across roofs of buildings, erect "poles"/"walls" by attaching thin plastic dowels to building walls (or, in non-urban areas, simply free-standing wooden stakes), and so on. (I only know some of the details because I've helped build such a monstrosity once or twice before.)
So this thing, together with some other legal elements, is called an eruv. Even though it's much less disruptive than a real wall (in fact, you wouldn't even notice one unless you knew how to look), and much less expensive too, it still usually requires city permission and it's still kind of pricey. The price goes up when you factor in the rabbi or other competent source required to not only certify that it was built correctly, but to have it checked every week to make sure it's still intact.
So a Center City (Philadelphia) eruv has been in the planning stages for years. It's had a website and brochures and a really ambitious map and everything. But no one knew how the process was coming along. Rumors were going around that it was anywhere from stalled to completed-but-not-certified. And then, suddenly, this weekend right before sundown Friday, the website changed and emails went out. The eruv was up!
Today for lunch, I went to a potluck picnic in the park.