A lens can only truly focus at one distance. Everything else forms not a point on the sensor but a little disc — the circle of confusion. While that disc stays smaller than about 0.03 mm the eye still reads it as sharp; the band of distances that clears the bar is the depth of field.
Two dials set how deep that band runs. A wider aperture (smaller f-number) lets the cone of light spread faster, so the disc grows quickly and the band shrinks. A longer focal length magnifies that same defocus, thinning the band further — which is why an 85 mm at f/1.8 melts everything but one face.
Stop the aperture down and the band deepens until, at the hyperfocal distance H = f²/(N·c), it reaches all the way to infinity. Focus there and the sharp zone runs from half that distance out forever — the trick that keeps a whole street crisp on a wide lens.
Nothing here is folklore: the blur you see is the exact CoC diameter, and the gold band is the depth of field solved from the same equation. Move an actor, pull focus, pinch the iris — the physics answers honestly.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.