"Occam's Razor" is a principle that states that the explanation of any phenomenon should make as few assumptions as possible, eliminating those that make no difference in the observable predictions of the explanatory hypothesis or theory.
Example: There's a tree down on the road. You can either say that...
Theory A: The huge storm last night knock the tree down.
Theory B: The huge storm last night caused a UFO to crash onto tree, knocking it down.
Layman's Term: Keep it simple, stupid.