A lens can only bring one distance to a perfect point. Everything at that distance forms a thin sheet in the world: the plane of focus. Here it is the glowing sheet sweeping through the valley, and the bright line where it cuts the trees, the cabin and the mountains.
Light from any point spreads out, passes through the whole opening of the lens and is bent back together. From a point on the plane, the rays meet exactly on the image plane: a point. From anything nearer or farther they meet in front of it or behind it, so they arrive as a small disc, the circle of confusion. That disc is the blur. The rings on the glass at the back show how big it is for the tree, the cabin and the peak.
Turning the ring slides the glass forward or back inside the barrel. Closer subjects focus farther behind the lens, so the glass moves out; far ones focus closer to it, so it moves in. Drag the ring, the slider on the console or the one in the card.
The iris is the adjustable hole in the middle of the lens. Wide open (f/2) the cone of light is fat, the discs are big and only a thin slice is sharp. Stopped down (f/16) the cone is thin, every disc shrinks, and the sharp zone grows deep. That is why landscapes are shot at f/11 and portraits at f/2.
Yes. Every lens projects the world upside down, and it is drawn that way on the glass. The screens on the console flip it back, the way your camera does.
1 2 3 focus on foreground, middle, background · [ ] nudge focus · F aperture · X assembled or exploded · C cinematic cameras · R reset the view · / hide the interface · drag to orbit · right-drag to pan · scroll to zoom · WASD to move, Q E to turn.
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Numbers are for a 50 mm lens on a full-frame camera; the valley is about 60 cm away.