The way this works: You have a blank PCB coated in a thin copper layer, and it's coated in a photoresist material. This material breaks down when exposed to light.
You then shine a bright (usually UV) light at it for a while, and the spots which got hit by light break down, while the other parts (which were in shadow) remain on the board.
You then use a corrosive chemical to etch away the copper layer, but it only etches away the parts that aren't covered in photoresist.