https://sael.net/plane-of-focus/
A lens can bring only one distance into perfect focus. Everything at that distance forms a thin sheet in the world, the plane of focus. Light from a point on that sheet passes through the whole lens and meets again at a single point on the sensor. Light from anything nearer or farther meets in front of the sensor or behind it, so it lands as a small blur disc, called the circle of confusion.
Turning the focus ring slides the glass forward or back, which moves the plane of focus nearer or farther. The aperture is the adjustable hole in the lens. Wide open, at f/2, the cone of light is wide, the blur discs are big and only a thin slice looks sharp. Stopped down to f/16, the cone narrows, every disc shrinks and the sharp zone, the depth of field, grows much deeper. That's why landscapes are often shot at f/11 and portraits at f/2.
The interactive lets you move the plane of focus through a small low-poly mountain scene with a cabin and pines, switch the aperture to watch the sharp zone change, and explode the lens to follow the light through each glass element to the upside-down image on the image plane.
Try it: The Plane of Focus