If you have a positive layer, you can create a positive mask. That means that the transparent film that the artwork gets printed on (for the photolithography tool), is opaque (black ink) where the copper/conductors/polygons will be. If you had a negative layer, then the mask would turn out being all black; except where the traces would be, it would be clear.
Positive and negative layers are identical in terms to the photography terms. In black & white photos, the negatives (developed film) were inverted from the colors on the final photograph.