Copper tube from water heater

There is a long copper tube leading from my water heater to a hot water pipe at the opposite end of the building. There it ends, having been unscrewed from the point where it connected to the pipe, and that connection capped off. I assume that the purpose of the tube is to allow for draining the water heater, but I'm not sure. Should I screw it back into the hot water pipe or leave it loose? If I screw it back in, what is to prevent the hot water from backflowing into the copper tube? Kind of a novice at this; I replaced much of my plumbing but I don't really understand what to do about this tube, or exactly what it is for. Thanks for any insight.

your photos are set on private

looks to be a circulating line, but it needs a pump

If that galvanized pipe is unscrewed, does water shoot out, or does it go to a drain line of some type? This will let you know if it is a recirc system or a drain line for the water tank.