What does focusing mean in photography?

Focusing is also called focusing, convex lens imaging. For an object at infinity, its imaging position is in the focus.

But there are few opportunities to shoot at infinity (such as shooting mountains and rivers), and most of them are objects within the range of 0 ~ 10 meters. At this time, the imaging position is not at the focus. Before focusing, in order to take such a close-up image, it is necessary to reduce the distance between the lens and the photosensitive element, which can be achieved by moving the position of the lens or the photosensitive element. In practical application, it is most convenient to move the lens assembly, so most lenses have focusing function.

PS, in the old cameras, you can focus by changing the position of the leather cavity, while cameras like Mamiya Six focus by moving the focal plane, and the lens does not move.