What Causes Light to Behave Differently in the Double-Slit Experiment?

[Flustered]

I just recently started studying things on the quantum level, I seen an experiment where they shot a beam of light through a hole and it appeared in many places, but when they observed in or something like that. It only beamed through one hole, can someone elaborate more on what that is and what is causing this to happen?

 

[Fredrik]

You should probably start with the Wikipedia article about the double-slit experiment, and then ask more specific questions.