It is known that an image is in focus (and it results sharp and clear) only at the focus, in which all rays intersect themselves. Why do we see sharp images only at the point in which the rays emitted of an object meet themselves?
Light rays converge to the points on the focal plane, and the diverge as they go past. Only at the focal plane are they concentrated to a sharp point; elsewhere they would look blurred out.
(From this answer to a related question)
If you put your sensor at the focus distance, all the yellow lines hit it at one point. Elsewhere, they would hit at many points, making a larger blurry yellow spot.