Occam's razor is a principle from philosophy.
Suppose one of the fence posts is broken and there are two explanations for the event:
a) A moose ran through it.
b) Some screws fell out of it because it is old.
According to Occam's razor, the simpler explanation is usually better. Simpler is better because the more assumptions you have to make, the more unlikely an explanation is. So, explanation b is the likelier explanation.