presensitised means the copper surface is coated with a layer of photosensitive material.
your printed layout must be on transparent 'paper'.
when you lay the printed transparency on the sensitised surface of the board, [after peeling off any protective layer], you then expose the board through the transparency.
where there is ink on the transparency, the board is not exposed to the light. where there are gaps, the board is exposed.
when you develop the board, the exposed areas are washed away, the unexposed areas that were under the ink are left intact.
after washing in water, etch the developed board in the etchant of choice/availability. the exposed areas will have the copper completely etched away, the areas shielded by the ink on the transparency will remain.
magazine paper is sometimes used in the 'toner transfer' method, using laser printer or photocopier.