A long wire is a simple antenna system used to communicate by radio. It's so simple, it's one of the first antennas that ham radio operators use to get on the air.
I used a long wire in my back yard when I first received my amateur radio license in the mid-1980's. I was still in the US Navy, and was stationed in Millington, Tennessee. I put up a long wire, connected to an antenna tuner, and finally over to my "new" Heathkit HW-101 tube transceiver, sometimes affectionately known as Hot Water 101's.
I tuned my radio to 28.400MHz, right in the middle of the 10-meter radio band. Since my license class was Novice, that was the only space where I could use voice communications. I turned on the radio and was immediately blasted by a radio station in Estonia, coming in loud and clear. I keyed my microphone and he came right back to me. That really started my love affair with Ham Radio.
The long wire was just that -- a long piece of copper wire zig-zagged around the yard. It was horribly inefficient, but it worked.
The better way to use a long wire antenna, especially on the lower ends of the amateur radio bands, is to use either a tuned wire, cut specifically for that band, or to install some traps, which allows one to operate several different bands with reasonable efficiency.
One of the issues with a long wire is to have the real estate to put one up on the lower bands. A 160-meter long wire, full wave, is 160 meters long, or 525 feet, or (for better visualization) 174 yards...the equivalent of one and three-quarters football fields long.
One of the benefits of a long wire is the extremely low take-off angle of the primary signal lobe. This makes communications around the world more reliable and you can reach better distances.