Depth of Field.
- The distances in front and behind the point of focus which are in acceptable focus .
- Dependant upon the focal length of the lens (shorter gives more) and the aperture. (smaller holes/higher numbers give more)
You're concerned about this when using Aperture Priority, and relying heavily on your Autofocus when you're shooting at Shutter-Speed Priority (sports &action pics)
In the very Olden Days, photogs would use charts, often their own making... Came the Industrial Revolution and things were mass - produced, so lens makers engraved them on the lenses...
The Digital Age saw them quietly dropped, mainly 'cos lenses became so (physically) small, and the operators had NO idea of what they were doing anyway...
Now we have smart Phone-Apps that calculate it for us, or print it out to carry around...
I'm used to the scales engraved on my lens-barrels...:p
D of F naturally leads on to the concept of 'Hyperfocal distance' - Google it for some useful & informative reading !
