Normally the lens of a camera and the sensor plane are parallel and so the plane of focus is parallel to the lens/sensor. When the lens is tilted, a plane that is not parallel to the image plane can also be completely in focus. When the image and sensor plane are not parallel, adjusting the focus actually rotates the plane of focus. A good example is shown in Figure 2 and 3 in the following link:
https://en.wikipedia.org/wiki/Scheimpflug_principle
Normally the lens of a camera and the sensor plane are parallel and so the plane of focus is parallel to the lens/sensor. When the lens is tilted, a plane that is not parallel to the image plane can also be completely in focus. When the image and sensor plane are not parallel, adjusting the focus actually rotates the plane of focus. A good example is shown in Figure 2 and 3 in the following link: https://en.wikipedia.org/wiki/Scheimpflug_principle