It was a network of some kind that provided automatic access to the OP to many ter'angreal, vehicles etc. - the OP equivalent of a power grid. It's the answer to a question that puzzled quite a few AS: how is it that non-channellers in the AOL are said in books to have been able to use ter'angreal?

How it worked exactly is unknown. It was probably very high tech devices, able to draw the OP (like the BOW drew saidin on its own once activated, while each ter'angreal properly included its own device to tap properly in the standing flows (a comment by Moridin even suggests AS may not have known how to operate manually most mundane ter'angreal, because they never had to in their Age)

As this was used for planes and ground vehicles as well as wireless devices (like callboxes and the pager of Alviarin), most people believe this worked more like cell phone towers, a series of devices creating a net of OP access over all areas of the world, rather than like electricity, needing to be transported to each home etc.

All we know beyond that is that the standing flows were disrupted at some point during the WOS.

The devices were most likely damaged in the war, and at some point, virtually all technological capacity was lost, so they couldn't be repaired. By the end of the war, we know most people had to do with horses and carts or their own feet to move around. Among other things, this implies that during the Breaking, most people were pretty much stuck where they were and in fairly squalid conditions. Imagine life in our big cities if we lost electricity and fuel overnight, and it won't be back. All our communications, means of transportation, most of our food industry, our water systems etc. would all be gone. Then to make it worse, add giant cataclysms on a regular basis and mass killing madmen into the mix.