Having been a professional photographer for 60 plus years, its a manner of focusing a manual camera to obtain the maximum depth of field.

That's technical jargon in optics.. you can look them up on the Wikipedia: https://en.wikipedia.org/wiki/Hyperfocal_distance

Here's a pretty good picture: https://tysonrobichaudphotography.files.wordpress.com/2011/07/img_5130-1.jpg This particular lens has been set to the hyperfocal distance for f/16, which appears to be about 14 meters, and everything from 7 meters to infinity will be in focus. The hyperfocal distance is different at different f/stops, and different between lenses of different focal lengths.

I have an old camera with manual focussing and aperture. If I set the aperture to f/5.6 and set the focus to 8 feet, then something at 8 feet will be in absolutely perfect focus. A little gadget shows me that the "depth of field" is from 4 feet to infinity. Theoretically the focus is perfect only at 8 feet, but everything from 4 feet to infinity will be in good enough focus. 8 feet is the "hyperfocal distance" at f/5.6 because the depth of field goes all the way out to infinity, and comes as close as possible. If I set it closer than 8 feet, then objects at infinity will be out of focus. If I set it farther away than 8 feet, then objects at 4 feet will be out of focus. It's the best compromise setting if you want everything in focus out to infinity. If I were taking pictures of kids running around in the backward and wanted to shoot quickly without taking time to focus, I'd set the camera to the hyperfocal distance, 8 feet, and just leave it there. I would know that as long as the kids didn't get any closer than eight feet, they would be in good focus, and so would everything else behind them in the picture.