The pattern represents the probability that a photon will strike. The bands are areas of high probability that a photon will strike there while the blank columns in between are the lowest probability that a photon will strike there, and it shows that photons have wave like properties, wave-particle duality blah blah blah. We were taught this in school and its also in the A+ notes book, its called 'Taylor's experiment'. Cant find a good website explaining it online, but that was with minimal googling.